________________ test_constraints_local_editable_install_pep518 ________________
    ...
    ----------------------------- Captured stdout call -----------------------------
    Script result: python -m pip download setuptools wheel -d /tmp/pytest-of-mockbuild/pytest-0/test_constraints_local_editabl0/data/packages
      return code: 1
    -- stderr: --------------------
      Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7fba7fdeb160>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/setuptools/
      Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7fba7fdeb0b8>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/setuptools/
      Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7fba7fdeb4e0>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/setuptools/
      Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7fba7fdeb6d8>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/setuptools/
      Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7fba7fdeb860>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/setuptools/
      Could not find a version that satisfies the requirement setuptools (from versions: )
    No matching distribution found for setuptools

    _____________ test_pep517_wheels_are_not_confused_with_other_files _____________
    ...
    -- stdout: --------------------
    Processing /tmp/pytest-of-mockbuild/pytest-0/test_pep517_wheels_are_not_con0/data/src/withpyproject
      Installing build dependencies: started
      Installing build dependencies: finished with status 'error'
      Complete output from command /tmp/pytest-of-mockbuild/pytest-0/test_pep517_wheels_are_not_con0/workspace/venv/bin/python /builddir/build/BUILDROOT/python-pip-19.0.2-1.fc30.x86_64/usr/lib/python3.7/site-packages/pip install --ignore-installed --no-user --prefix /tmp/pytest-of-mockbuild/pytest-0/test_pep517_wheels_are_not_con0/workspace/tmp/pip-build-env-3un7dqu3/overlay --no-warn-script-location --no-binary :none: --only-binary :none: -i https://pypi.org/simple -- setuptools wheel:
      Collecting setuptools
        Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7fb7cf5a8898>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/setuptools/
        Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7fb7cf5b9588>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/setuptools/
        Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7fb7cf5b9470>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/setuptools/
        Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7fb7cf5b9278>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/setuptools/
        Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7fb7cf5b9208>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/setuptools/
        Could not find a version that satisfies the requirement setuptools (from versions: )
      No matching distribution found for setuptools

    ________________________ test_upgrade_argparse_shadowed ________________________
    ...
    ----------------------------- Captured stdout call -----------------------------
    Script result: python -m pip install argparse==1.3
      return code: 1
    -- stderr: --------------------
      Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7ff07d6db240>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/argparse/
      Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7ff07d6db358>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/argparse/
      Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7ff07d6db400>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/argparse/
      Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7ff07d6db518>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/argparse/
      Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7ff07d6c7710>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/argparse/
      Could not find a version that satisfies the requirement argparse==1.3 (from versions: )
    No matching distribution found for argparse==1.3

The main setuptools PEP 517 backend is intended for
explicit usage in `pyproject.toml`, when the project
authors can ensure that their `setup.py` runs without
that directory being implicitly on `sys.path`.

For implicit usage, setuptools now offers a separate
legacy backend that more closely mimics direct
execution of the `setup.py` script.

The message looks like this:

  File "setup.py" not found. Directory cannot be installed in editable
  mode: <absolute-dir-path>
  (A "pyproject.toml" file was found, but editable mode currently
  requires a setup.py based build.)

pip usually emits a warning and tests have to be adapted

Fix test_pep518_with_user_pip which was getting errors due to irrelevant changes in setuptools output

- check build requirements for conflicts
- better isolation (ignore system site packages)
- support 2 prefixes: a "normal" one, and an "overlay" one
  (with higher priority over "normal")

- cleanup virtualenv creation code
- ensure all testing virtual environments use a recent version
  of setuptools / wheel, making it easier to switch to custom
  versions of those, as well as reducing network accesses
- reduce size of testing virtual environment, slightly speeding
  up the testsuite

Handle .pth files, so namespace packages are correctly supported under Python 3.2 and earlier, and more.

Ensure the subprocess call to pip for installing the PEP 518
build dependencies is using the same version of pip.

As spelled on https://pypi.org/.

* Move dist restriction options to be re-usable (between install/download)
* Make dist restriction options usable in `install` (exclusively with --target)
* Add a check_supported_wheels bool to RequiriementSet for non-resolved (full path) dependencies

No need for a wheel of `simple` now that build dependencies support source installs.