Use guard clause in VersionControl.check_destination().

get-pip: add notes to docs for specifying versions

get-pip.py supports installing specified versions of pip, wheel, setuptools...
and any dependency for that matter, but this is not clearly exposed in
the documentation at https://pip.pypa.io/en/stable/installing/#installing-with-get-pip-py

Improve readability of version checking caller

Creating a news entry from the pull request as a separate commit.  Does this warrant an issue?

Revert "Skip AppVeyor builds on non-code changes"

Skip AppVeyor builds on non-code changes

Remove the old get-pip.py shim

Add a new tools/ directory and move some files into it

Start warning on non compliant pyproject.toml files and rejecting obviously incorrect ones

Update doc and comment URLs to https where available

Document the output format of pip show

The installed set of packages is already fixed and no longer changing.
It is therefore safe to reuse a single `pkg_resources.WorkingSet` for
all invocations of `get_installed_version`.

Restrict install time dependency warnings to directly-dependant packages

Reorder methods in req_install to better group them

Remove useless manifest entries

These were made redundant since those files have been removed.