[red-knot] Change venv discovery (#17099)

## Summary

Rewrites the virtual env discovery to:

* Only use of `System` APIs, this ensures that the discovery will also
work when using a memory file system (testing or WASM)
* Don't traverse ancestor directories. We're not convinced that this is
necessary. Let's wait until someone shows us a use case where it is
needed
* Start from the project root and not from the current working
directory. This ensures that Red Knot picks up the right venv even when
using `knot --project ../other-dir`

## Test Plan

Existing tests, @ntBre tested that the `file_watching` tests no longer
pick up his virtual env in a parent directory

crates/red_knot_python_semantic/src/program.rs

@@ -145,7 +145,8 @@ pub enum PythonPath {
     /// [`sys.prefix`]: https://docs.python.org/3/library/sys.html#sys.prefix
     SysPrefix(SystemPathBuf, SysPrefixPathOrigin),
 
-    Discover,
+    /// Tries to discover a virtual environment in the given path.
+    Discover(SystemPathBuf),
 
     /// Resolved site packages paths.
     ///