Write the path of the parent environment to an extends-environment key in the pyvenv.cfg file of an ephemeral environment

[AlexWaygood]

Summary

Ephemeral environments created by uv run --with extend a parent (virtual or system) environment by adding a .pth file to the ephemeral environment's site-packages directory. The pth file contains Python code to dynamically add the parent environment's site-packages directory to Python's import search paths in addition to the ephemeral environment's site-packages directory. This works well at runtime, but is not currently understood by static analysis tools like ty.

As discussed in astral-sh/ty#320, ty could implement support for these virtual environments by attempting to parse the Python code uv is writing into these .pth files. This has many disadvantages however:

• It seems somewhat error-prone. The specific Python code uv is writing into the .pth files could change at any time -- and if another tool implements a similar ephemeral-environment feature, it could use different Python idioms.
• It would not be particularly efficient for ty to have to attempt to parse Python snippets in every .pth file in the site-packages directory (it could hurt our ability to effectively implement a persistent cache even if it doesn't cost that much in wall time, since we need to know the import search paths very early on).
• Philosophically, it feels out of spirit with the kind of analysis a static type checker should be expected to do.

Instead, this PR implements a feature where uv additionally writes the sys.prefix of the parent environment to a new extends-environment key of the ephemeral environment's pyvenv.cfg file. This should make it much easier for ty -- and potentially other static-analysis tools -- to statically and reliably understand the relationship between the two environments. If ty sees that a pyvenv.cfg file has a uv key in it and also has a extends-environment key in it, it will be able to easily add the site-packages directory of the parent environment to its resolved search paths as well as the site-packages directory of the ephemeral child environment.

A question was raised in astral-sh/ty#320 (comment) about whether it was standards-compliant to add new keys to pyvenv.cfg files created by uv. I think it is. pyvenv.cfg files are in general very under-specified: only a small handful of keys are currently consistent between virtual-environment creation tools. Different tools already often add bespoke keys to their pyvenv.cfg files -- see astral-sh/ty#320 (comment) for some comparisons between uv, the virtualenv Python library, and the standard-library venv module.

Test Plan

I added a test that demonstrates that the extends-environment key is written with the correct value to the pyvenv.cfg file of an ephemeral environment.

[zanieb]

As a note, this will suffer from the same issue as reported in #12889 — but it won't be "worse".

[AlexWaygood]

Argh, well, it seems I need some help with getting the venv directory filtering to work on Windows. (I haven't actually changed anything to do with the filtering there -- not sure why it's not working on this snapshot?)

Other than that, this is ready for review!

[zanieb]

(I haven't actually changed anything to do with the filtering there -- not sure why it's not working on this snapshot?)

I don't think we've ever filtered escape_for_python paths (which I guess differ on Windows).

[AlexWaygood]

Well, the test is finally passing on all platforms... If there's a better way to write a test for this, LMK!