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

[jaraco]

Fixes #2355.

[jaraco]

I tested this patch by installing setuptools from the branch, then running it against one of the projects that failed with SETUPTOOLS_USE_DISTUTILS=local:

$ env SETUPTOOLS_USE_DISTUTILS=local .tox/python-native/bin/pip install hg+http://hg.code.sf.net/p/ruamel-yaml-clib/code@0.2.0
Collecting hg+http://hg.code.sf.net/p/ruamel-yaml-clib/code@0.2.0
  Cloning hg http://hg.code.sf.net/p/ruamel-yaml-clib/code (to revision 0.2.0) to /private/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-req-build-hmnmb0ru
  Running command hg clone --noupdate -q http://hg.code.sf.net/p/ruamel-yaml-clib/code /private/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-req-build-hmnmb0ru
  *** failed to import extension mercurial_keyring: No module named 'mercurial_keyring'
  *** failed to import extension hggit from ~/.hgext/hg-git/hggit: No module named 'dulwich'
  *** failed to import extension git-tags from ~/.hgext/git-tags.py: ('tag', '[b'debugancestor', b'debugantivirusrunning', b'debugapplystreamclonebundle', b'debugbuilddag', b'debugbundle', b'debugcapabilities', b'debugcheckstate', b'debugcolor', b'debugcreatestreamclonebundle', b'debugdag', b'debugdata', b'debugdate', b'debugdeltachain', b'debugdirstate', b'debugstate', b'debugdiscovery', b'debugdownload', b'debugextensions', b'debugfileset', b'debugformat', b'debugfsinfo', b'debuggetbundle', b'debugignore', b'debugindex', b'debugindexdot', b'debugindexstats', b'debuginstall', b'debugknown', b'debuglabelcomplete', b'debuglocks', b'debugmanifestfulltextcache', b'debugmergestate', b'debugnamecomplete', b'debugnodemap', b'debugobsolete', b'debugp1copies', b'debugp2copies', b'debugpathcomplete', b'debugpathcopies', b'debugpeer', b'debugpickmergetool', b'debugpushkey', b'debugpvec', b'debugrebuilddirstate', b'debugrebuildstate', b'debugrebuildfncache', b'debugrename', b'debugrequires', b'debugrequirements', b'debugrevlog', b'debugrevlogindex', b'debugrevspec', b'debugserve', b'debugsetparents', b'debugsidedata', b'debugssl', b'debugbackupbundle', b'debugsub', b'debugsuccessorssets', b'debugtagscache', b'debugtemplate', b'debuguigetpass', b'debuguiprompt', b'debugupdatecaches', b'debugupgraderepo', b'debugwalk', b'debugwhyunstable', b'debugwireargs', b'debugwireproto', b'abort', b'add', b'addremove', b'annotate', b'blame', b'archive', b'backout', b'bisect', b'bookmarks', b'bookmark', b'branch', b'branches', b'bundle', b'cat', b'clone', b'commit', b'ci', b'config', b'showconfig', b'debugconfig', b'continue', b'copy', b'cp', b'debugcommands', b'debugcomplete', b'diff', b'export', b'files', b'forget', b'graft', b'grep', b'heads', b'help', b'identify', b'id', b'import', b'patch', b'incoming', b'in', b'init', b'locate', b'log', b'history', b'manifest', b'merge', b'outgoing', b'out', b'parents', b'paths', b'phase', b'pull', b'push', b'recover', b'remove', b'rm', b'rename', b'move', b'mv', b'resolve', b'revert', b'rollback', b'root', b'serve', b'shelve', b'status', b'st', b'summary', b'sum', b'tag', b'tags', b'tip', b'unbundle', b'unshelve', b'update', b'up', b'checkout', b'co', b'verify', b'version']')
  Running command hg update -q 0.2.0
  *** failed to import extension mercurial_keyring: No module named 'mercurial_keyring'
  *** failed to import extension hggit from ~/.hgext/hg-git/hggit: No module named 'dulwich'
  *** failed to import extension git-tags from ~/.hgext/git-tags.py: ('tag', '[b'debugancestor', b'debugantivirusrunning', b'debugapplystreamclonebundle', b'debugbuilddag', b'debugbundle', b'debugcapabilities', b'debugcheckstate', b'debugcolor', b'debugcreatestreamclonebundle', b'debugdag', b'debugdata', b'debugdate', b'debugdeltachain', b'debugdirstate', b'debugstate', b'debugdiscovery', b'debugdownload', b'debugextensions', b'debugfileset', b'debugformat', b'debugfsinfo', b'debuggetbundle', b'debugignore', b'debugindex', b'debugindexdot', b'debugindexstats', b'debuginstall', b'debugknown', b'debuglabelcomplete', b'debuglocks', b'debugmanifestfulltextcache', b'debugmergestate', b'debugnamecomplete', b'debugnodemap', b'debugobsolete', b'debugp1copies', b'debugp2copies', b'debugpathcomplete', b'debugpathcopies', b'debugpeer', b'debugpickmergetool', b'debugpushkey', b'debugpvec', b'debugrebuilddirstate', b'debugrebuildstate', b'debugrebuildfncache', b'debugrename', b'debugrequires', b'debugrequirements', b'debugrevlog', b'debugrevlogindex', b'debugrevspec', b'debugserve', b'debugsetparents', b'debugsidedata', b'debugssl', b'debugbackupbundle', b'debugsub', b'debugsuccessorssets', b'debugtagscache', b'debugtemplate', b'debuguigetpass', b'debuguiprompt', b'debugupdatecaches', b'debugupgraderepo', b'debugwalk', b'debugwhyunstable', b'debugwireargs', b'debugwireproto', b'abort', b'add', b'addremove', b'annotate', b'blame', b'archive', b'backout', b'bisect', b'bookmarks', b'bookmark', b'branch', b'branches', b'bundle', b'cat', b'clone', b'commit', b'ci', b'config', b'showconfig', b'debugconfig', b'continue', b'copy', b'cp', b'debugcommands', b'debugcomplete', b'diff', b'export', b'files', b'forget', b'graft', b'grep', b'heads', b'help', b'identify', b'id', b'import', b'patch', b'incoming', b'in', b'init', b'locate', b'log', b'history', b'manifest', b'merge', b'outgoing', b'out', b'parents', b'paths', b'phase', b'pull', b'push', b'recover', b'remove', b'rm', b'rename', b'move', b'mv', b'resolve', b'revert', b'rollback', b'root', b'serve', b'shelve', b'status', b'st', b'summary', b'sum', b'tag', b'tags', b'tip', b'unbundle', b'unshelve', b'update', b'up', b'checkout', b'co', b'verify', b'version']')
Requirement already satisfied (use --upgrade to upgrade): ruamel.yaml.clib==0.2.0 from hg+http://hg.code.sf.net/p/ruamel-yaml-clib/code@0.2.0 in ./.tox/python-native/lib/python3.8/site-packages
Building wheels for collected packages: ruamel.yaml.clib
  Building wheel for ruamel.yaml.clib (setup.py) ... done
  Created wheel for ruamel.yaml.clib: filename=ruamel.yaml.clib-0.2.0-cp38-cp38-macosx_10_9_x86_64.whl size=156596 sha256=74f64005c59d271a420d04a7c00ce3b6c05ed6aa55398c170255fbf077f8ef52
  Stored in directory: /private/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-ephem-wheel-cache-az3npo63/wheels/61/aa/60/dfd8fbec3bf6369be553cde8e9307ed257adce7b7b00719ac3
Successfully built ruamel.yaml.clib
WARNING: You are using pip version 20.1.1; however, version 20.2.2 is available.
You should consider upgrading via the '/Users/jaraco/code/public/pypa/setuptools/.tox/python-native/bin/python -m pip install --upgrade pip' command.