Support importing across repos with bzlmod

# 🚀 feature request

### Relevant Rules
`py_library` / `py_binary`

### Description

If there is a python library `@foo//bar:baz`, how can I import it from the main repository?

Pre-bzlmod, you could write `from foo.bar import baz`, as it would be in the directory `external/foo/bar/baz`, and the external directory was in your python path. Apparently this was not documented or intended, but nevertheless, people used it, as I don't believe it was ever considered when writing rules_python.

In bzlmod, however, you have repo mapping, where the directory `foo` is actually stored at `external/_main~~foo_ext~foo`, and thus `from foo.bar import baz` fails. This is the direct cause of #1679, and if we solve this, we fix that bug for free.

### Describe the solution you'd like
I'd personally like to make users prefix their imports to external repositories with the name of the workspace (based on the repo mapping). This would allow two python files in different repositories to `import foo.bar.baz` and each get their own `@foo` in the case of a repo name clash (this is how repo mapping is intended to be used, to allow each repo to have their own namespace of repos).

I previously achieved this in ChromeOS via a `sitecustomize.py` (see discussion in #1679).

### Describe alternatives you've considered

I can't really think of a viable alternative. Any solution without including the repo name in the import seems like it will end up in painful corner cases where you have name conflicts.