Merge pull request #2381 from pypa/bugfix/2355-keep-distutils-patched-when-pip-imported-during-build

When pip is imported as part of a build, leave distutils patched.

Reduce size of setuptools' bdist_rpm._make_spec_file

There are some setuptools specific changes in the bdist_rpm module that
are no longer needed, because the upstream/shipped version of distutils
already contains them. The code that is removed in this commit from
bdist_rpm is already part of the python-3.5 version of distutils.

Related: #2377

Make stdlib distutils the default again. Stop the burning.

Restore pythonlib support on older Pythons.

In distutils hack, use absolute import rather than relative to avoid bpo-30876.

Add Python 3.10 support to _distutils_hack

Get the 'Loader' abstract class from importlib.abc rather than
importlib.util.abc (alias removed in Python 3.10).

Supply a copy of _optim_args_from_interpreter_flags for Python 3.5 compatibility. Ref pypa/setuptools#2357.

Re-enable distutils patch by default.

Resolving error text issue (#2302)