Allow using a locally installed Python runtime as a full toolchain

This feature request has come up a few time from different places.

The basic request is to have an option other than the hermetic runtimes, typically a Python runtime that is locally installed. Some of the reasons this is appealing:

* Avoids having to download a remote runtime.
* If you deeply care about supply chain issues, the provenance of the runtimes isn't very compelling.
* The standalone-python-build runtimes have some known limitations
* The hermetic runtimes are expensive to setup: they're inbuild runtimes with a (mostly) full python installation, all of which has to be copied into runfiles
* I think some people like to manage their Python version outside of bazel, e.g. using `pyenv` or venv activation.

Making a local runtime have the same capabilities as a hermetic runtime is _sort of_ possible today, but requires quite a bit of setup, and I think maybe loading a few private bzl files.

PR #2000 lays the ground work for most of it. What's now missing is a public API for it and figuring out what knobs (if any) it needs.

For at least early testing, we're thinking to have a flag control what type of runtime is selected.

But, during our maintainers meeting, two big questions came up:
1. What does the public API for using it look like?
2. What are the use cases? These drive what the public API needs to look like

So, if you have use cases, please post them here. It'll help us figure out what capabilities are needed.

TODO
* [x] Create repo rules to map the local python install into bazel
* [x] Allow setting arbitrary target_compatible_with/target_settings
* [ ] Allow creating an in-build instead of platform runtime

Start by reviewing PR #2000 and the completed TODO items for mapping a local Python installation into Bazel. Define the public API and required knobs from the documented use cases, then determine how to support creating an in-build rather than platform runtime.

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