mkvenv: use pip's vendored distlib as a fallback

distlib is usually not installed on Linux distribution, but it is vendored
into pip.  Because the virtual environment has pip via ensurepip, we
can piggy-back on pip's vendored version.  This could break if they move
our cheese in the future, but the fix would be simply to require distlib.

If it is debundled, as it is on msys, it is simply available directly.

Signed-off-by: John Snow <jsnow@redhat.com>
[Move to toplevel. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

python/scripts/mkvenv.py

@@ -69,10 +69,25 @@
 import venv
 import warnings
 
-import distlib.database
-import distlib.scripts
-import distlib.version
 
+# Try to load distlib, with a fallback to pip's vendored version.
+# HAVE_DISTLIB is checked below, just-in-time, so that mkvenv does not fail
+# outside the venv or before a potential call to ensurepip in checkpip().
+HAVE_DISTLIB = True
+try:
+    import distlib.database
+    import distlib.scripts
+    import distlib.version
+except ImportError:
+    try:
+        # Reach into pip's cookie jar.  pylint and flake8 don't understand
+        # that these imports will be used via distlib.xxx.
+        from pip._vendor import distlib
+        import pip._vendor.distlib.database  # noqa, pylint: disable=unused-import
+        import pip._vendor.distlib.scripts  # noqa, pylint: disable=unused-import
+        import pip._vendor.distlib.version  # noqa, pylint: disable=unused-import
+    except ImportError:
+        HAVE_DISTLIB = False
 
 # Do not add any mandatory dependencies from outside the stdlib:
 # This script *must* be usable standalone!
@@ -664,6 +679,10 @@ def ensure(
         bellwether for the presence of 'sphinx'.
     """
     print(f"mkvenv: checking for {', '.join(dep_specs)}", file=sys.stderr)
+
+    if not HAVE_DISTLIB:
+        raise Ouch("a usable distlib could not be found, please install it")
+
     try:
         _do_ensure(dep_specs, online, wheels_dir)
     except subprocess.CalledProcessError as exc:

python/setup.cfg

@@ -125,6 +125,24 @@ ignore_missing_imports = True
 [mypy-distlib.version]
 ignore_missing_imports = True
 
+[mypy-pip]
+ignore_missing_imports = True
+
+[mypy-pip._vendor]
+ignore_missing_imports = True
+
+[mypy-pip._vendor.distlib]
+ignore_missing_imports = True
+
+[mypy-pip._vendor.distlib.database]
+ignore_missing_imports = True
+
+[mypy-pip._vendor.distlib.scripts]
+ignore_missing_imports = True
+
+[mypy-pip._vendor.distlib.version]
+ignore_missing_imports = True
+
 [pylint.messages control]
 # Disable the message, report, category or checker with the given id(s). You
 # can either give multiple identifiers separated by comma (,) or put this