Search PATH when python can't be found with py

[MichaReiser]

Summary

This PR improves uv's compatibility with PIP when discovering python installations.

PIP runs the following step

• if -p is a path, resolve that specific instance
• test if the current python version sys.executable matches the requested version (or if no version was requested, always use it)
• (windows): Use PEP514 to resolve the python version
• Iterate over the paths and for each path test for python{major}.{minor}.{patch}, python{major}.{minor}, python{major}, python

Our existing implementation already did some of that but not all. We now

• search for executables that match the requested version name: python3.9 python3.9.3 for Windows and Unix
• Search the path on windows (rather than just relying on py)
• Support pyenv shims (that's a hack)
• Filter out the windows store python shim.

I merged most of the implementation between windows and unix because they are the same in venv too. The only real difference is that Windows must support py (PEP514) and shims are awkward.

Fixes #1310
Fixes #1660
Fixes #1168

Test Plan

• Verified that running uv venv on Windows with no Python installed but the Windows shims activate fails with an error that Python wasn't found rather than invoking the shim
• Verified that running uv venv with a local pyenv environment activate selects that Python version (because it is the first in the path)
• If there's a python.exe on path and the python.bat shim, then the installation takes the python.exe (avoid indirection) except if the python.exe doesn't satisfy the requested version.
• Verified on MacOs that uv venv picks up the different python versions (including pyenv)

[MichaReiser]

I intentionally removed the TEST early return. We want that as much code as possible in this function is run during tests.

[konstin]

Can request be empty? I just realized then we could hit this path

[MichaReiser]

pub fn main() {
    let versions = ""
        .splitn(3, '.')
        .map(str::parse::<u8>)
        .collect::<Result<Vec<_>, _>>();

    dbg!(versions);
}

gives me

[src/main.rs:8:9] versions = Err(
    ParseIntError {
        kind: Empty,
    },
)

So we're good

[konstin]

When do we see a path multiple times?

[MichaReiser]

I copied that over from virtualenv but I'm not sure. Let's remove it for now.