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/module_resolver/resolver.rs

@@ -222,8 +222,14 @@ impl SearchPaths {
         static_paths.push(stdlib_path);
 
         let site_packages_paths = match site_packages_paths {
-            SitePackages::Derived { venv_path } => VirtualEnvironment::new(venv_path, system)
-                .and_then(|venv| venv.site_packages_directories(system))?,
+            SitePackages::Derived { venv_path } => {
+                // TODO: We may want to warn here if the venv's python version is older
+                //  than the one resolved in the program settings because it indicates
+                //  that the `target-version` is incorrectly configured or that the
+                //  venv is out of date.
+                VirtualEnvironment::new(venv_path, system)
+                    .and_then(|venv| venv.site_packages_directories(system))?
+            }
             SitePackages::Known(paths) => paths
                 .iter()
                 .map(|path| canonicalize(path, system))