[red-knot] Discover local venv folder in cli (#16917)

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## Summary

Fixes #16744 

Code from 

bbf4f830b5/crates/uv-python/src/virtualenv.rs (L124-L144)

## Test Plan

Manual testing

---------

Co-authored-by: Micha Reiser <micha@reiser.io>

crates/red_knot_python_semantic/src/program.rs

@@ -145,6 +145,8 @@ pub enum PythonPath {
     /// [`sys.prefix`]: https://docs.python.org/3/library/sys.html#sys.prefix
     SysPrefix(SystemPathBuf, SysPrefixPathOrigin),
 
+    Discover,
+
     /// Resolved site packages paths.
     ///
     /// This variant is mainly intended for testing where we want to skip resolving `site-packages`