Change EnvironmentOptions::venv-path to Option<SystemPathBuf> (#15631)

## Summary

The `Options` struct is intended to capture the user's configuration
options but
`EnvironmentOptions::venv_path` supports both a `SitePackages::Known`
and `SitePackages::Derived`.

Users should only be able to provide `SitePackages::Derived`—they
specify a path to a venv, and Red Knot derives the path to the
site-packages directory. We'll only use the `Known` variant once we
automatically discover the Python installation.

That's why this PR changes `EnvironmentOptions::venv_path` from
`Option<SitePackages>` to `Option<SystemPathBuf>`.

This requires making some changes to the file watcher test, and I
decided to use `extra_paths` over venv path
because our venv validation is annoyingly correct -- making mocking a
venv rather involved.

## Test Plan

`cargo test`

crates/red_knot_python_semantic/src/program.rs

@@ -134,12 +134,3 @@ pub enum SitePackages {
     /// Resolved site packages paths
     Known(Vec<SystemPathBuf>),
 }
-
-impl SitePackages {
-    pub fn paths(&self) -> &[SystemPathBuf] {
-        match self {
-            SitePackages::Derived { venv_path } => std::slice::from_ref(venv_path),
-            SitePackages::Known(paths) => paths,
-        }
-    }
-}