fix PEP 518 support when pip is installed in the user site

Reorder conditionals to deal with installed VCS requirements

Document limitations of current implementation of PEP 518 in pip

Update distlib to 0.2.7

Improve Error Messages on EnvironmentErrors when installing

Make the Windows "self-modification" check more strict

This fixes various issues with the current logic and splits out the
message generation into a separate unit-test-able function.

Improvements:
- Does not not-print any information in some cases
- Mentions original error message, or entire traceback if verbose.
- In case of permission/access errors, suggests the user to use --user
  and/or check permissions

Upgrade to sphinx 1.7

Fix msgpack on older versions of Python 2.7

Do not use the build environment isolation during the build dependencies installation.

Switch pip to use Warehouse by default

Fix testsuite virtualenv

Fix sites ordering: add user site first, before system site.

Ensure `virtualenv.system_site_packages = True` (needed for testing
`--user`) does not result in adding the real site-packages' directory
to `sys.path`.

Release 10.0.0

News file formatting fix

Don't warn about script locations if --target is specified

fix PEP 518 support