rules_python_external integration: drop bazel-mypy-integration - #353

Description

Removing bazel-mypy-integration from experimental/rules_python_external as the integration is not required at runtime by rules_python_external and keeping it in the project will make integration with the 'core' more complicated than necessary.

Plan 🛣

Our plan is in multiple steps:

• bring in the repo to a subdirectory, preserving git history and contributors
• clean up the code, remove mypy, get it functional in that subdirectory ← This PR is here
• add an example/integration test proving that it works
• cut a patch release and get some users to test it out
• communicate our deprecation plan to rules_python users
• BIG SWITCH: move the rules_python_external implementation to the root, make a legacy/ directory for current impl
• fill in any missing docstrings and include new rules in stardoc
• cut a breaking release (a minor, since semver starting with 0. states that minors are breaking)
• help users switch over
• in a later breaking release, actually remove the old implementation

PR Checklist

Please check if your PR fulfills the following requirements:

• Does not include precompiled binaries, eg. .par files. See CONTRIBUTING.md for info
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Does this PR introduce a breaking change?

• Yes
• No