refactor(pypi): translate wheel METADATA parsing to starlark (bazel-contrib#2629)

This PR starts using the newly introduced (bazel-contrib#2692) PEP508 compliant
requirement marker parser in starlark and moves the dependency
generation from the Python language (`whl_installer`) to the Starlark
in the `whl_library` repository rule.

This PR is (almost) a pure refactor where no bugs are fixed, but this is
foundational work that also adds notes on how things will be moved
to macros (i.e. analysis phase) so that we can fix a few long standing
bugs and prepare for stabilizing the `experimental_index_url` (bazel-contrib#260).

Refactor:
* I have migrated all of the unit tests from Python to starlark for deps
  generation from METADATA `Requires-Dist` fields.
* Read the `METADATA` file itself in Starlark.

Work towards bazel-contrib#260, bazel-contrib#2319, bazel-contrib#2241
Fixes bazel-contrib#2423

fix(toolchain) Override coverage rc

Author: aignas

Diff for: python/private/stage2_bootstrap_template.py

@@ -309,29 +309,28 @@ def _maybe_collect_coverage(enable):
 relative_files = True
 source =
 \t{source}
+omit = 
+            # Pipes can't be read back later, which can cause coverage to
+            # throw an error when trying to get its source code.
+           /dev/fd/* 
+            # The mechanism for finding third-party packages in coverage-py
+            # only works for installed packages, not for runfiles. e.g:
+            #'$HOME/.local/lib/python3.10/site-packages',
+            # '/usr/lib/python',
+            # '/usr/lib/python3.10/site-packages',
+            # '/usr/local/lib/python3.10/dist-packages'
+            # see https://github.com/nedbat/coveragepy/blob/bfb0c708fdd8182b2a9f0fc403596693ef65e475/coverage/inorout.py#L153-L164
+           */external/*
 """
         )
     try:
         cov = coverage.Coverage(
-            config_file=rcfile_name,
+            config_file=os.environ.get("COVERAGE_RCFILE", rcfile_name),
             branch=True,
             # NOTE: The messages arg controls what coverage prints to stdout/stderr,
             # which can interfere with the Bazel coverage command. Enabling message
             # output is only useful for debugging coverage support.
             messages=is_verbose_coverage(),
-            omit=[
-                # Pipes can't be read back later, which can cause coverage to
-                # throw an error when trying to get its source code.
-                "/dev/fd/*",
-                # The mechanism for finding third-party packages in coverage-py
-                # only works for installed packages, not for runfiles. e.g:
-                #'$HOME/.local/lib/python3.10/site-packages',
-                # '/usr/lib/python',
-                # '/usr/lib/python3.10/site-packages',
-                # '/usr/local/lib/python3.10/dist-packages'
-                # see https://github.com/nedbat/coveragepy/blob/bfb0c708fdd8182b2a9f0fc403596693ef65e475/coverage/inorout.py#L153-L164
-                "*/external/*",
-            ],
         )
         cov.start()
         try: