Convert tracing warnings during `site-packages` discovery to diagnostic warnings

[AlexWaygood]

This file has lots of places where it emits tracing warnings if site-packages discovery doesn't go to plan: https://github.com/astral-sh/ruff/blob/main/crates/ty_python_semantic/src/site_packages.rs. Ideally we should convert these to warning-level diagnostics so that they're more visible to IDE users (suggested by @MichaReiser in astral-sh/ruff#18335 (comment))

[MichaReiser]

To clarify: we should only convert the once that require user interaction (where we expect them to change something) because warning diagnostics can change the exit code

[AlexWaygood]

Hrm. A message like this I think would be useful for the user to see (it'll help them diagnose the cause of type-checking diagnostics) but I don't know what we expect the user to do about it. It most likely indicates a bug in uv or ty, if it happens! But I don't think we should fail the process, because type-checking might proceed just fine.

What about an info-level diagnostic? Would we still exit with code 0 if we did that?

[AlexWaygood]

I guess one action the user could take in response to that diagnostic is to not use uv run --with=ty, and instead run ty using some other uv invocation.