Include external srcs for mypy dep analysis with pip_install

[jonmeow]

At present, because external deps are skipped, requirement()-based imports don't work. This causes issues for python rules that are trying to use pip_install.

This makes a series of changes in order to enable requirement() compatibility:

• Adds include_imports, defaulting False, to enable new behavior.
  • I'm cautious of causing issues for those who don't need/want this. In my personal repo, tests go from 5s to 50s -- parsing of the pip imports is definitely slow.
  • The flipside is that parsing imports makes this behave consistently for people who do or don't have the imports locally installed, and that consistency is why I'm proposing this change. That is, we'll accept the slowdown for consistency.
• When extracting transitive deps, include_imports follows external and also gathers imports.
• If imports were found, does work to make them relative to the workspace root.
  • A mypy_test //foo:bar would run from the foo directory; ../ is added to imports in that case so that they're found correctly. The resulting mypy path will look like ../py_deps/pypi__pygithub, allowing import github to resolve to ../py_deps/pypi__pygithub/github.
• The target_src_files versus input_src_files split is done because target_src_files is the small set of project-owned files which are valid to examine, whereas input_src_files is used for runfiles generation and so needs to include imported files.
• --follow-imports=silent is added to avoid printing errors on imported files.
  • Can't fully ignore file with syntax error generated from mypy-protobuf library python/mypy#6897 prevents this from ignoring all type errors, in particular type hints from more recent versions of Python than supported.
  • --exclude may work better in the future (because it could be given the list of import paths), but isn't available at the current version of mypy in use by this rule.

This might fix #39, and may also help with #23 (although this may not interact correctly -- smarter import handling may be needed, this is really requirement()-focused).

[alexeagle]

This might fix #39

I patched this on top of the repro, but it's still red: https://github.com/bazel-contrib/bazel-mypy-integration/runs/6323849357

I think this is because it still doesn't place the stubs in a site-packages folder, which is the only place (?) mypy will look for it, as @rogerhub said in #39:

Mypy also offers a MYPYPATH, but it does not use the "...-stubs" layout, so we can't just stick all of the Bazel PyPI directories into the MYPYPATH (I tried that initially).