Add CLI flag to override typeshed

[connernilsen]

I forgot to add a --typeshed-like flag to pyrefly check, which will let us override typeshed. This is useful for testing typeshed upgrades, testing how changes to typeshed will affect a codebase, and other typeshed-related changes.

This will involve a couple of steps. For whomever takes this on, I can turn this into more sub-tasks if that would make it easier for you!

1. Add a CLI flag somewhere around
   

   pyrefly/pyrefly/lib/commands/check.rs

   Line 161 in cfb9561

   struct ConfigOverrideArgs {

2. Add a ConfigFile option for typeshed path
   

   pyrefly/pyrefly/lib/config/config.rs

   Line 164 in cfb9561

   pub struct ConfigFile {

3. Wire it into our find_import() logic
   

   pyrefly/pyrefly/lib/config/config.rs

   Lines 342 to 346 in cfb9561

   	pub fn find_import(
   	&self,
   	module: ModuleName,
   	path: Option<&Path>,
   	) -> Result<ModulePath, FindError> {

4. You'l likely have to update our typeshed functionality (all in the file below) to handle loading a custom typeshed
   

   pyrefly/pyrefly/lib/module/bundled.rs

   Lines 124 to 129 in cfb9561

   	pub fn typeshed() -> anyhow::Result<&'static BundledTypeshed> {
   	match &*BUNDLED_TYPESHED {
   	Ok(typeshed) => Ok(typeshed),
   	Err(error) => Err(anyhow!("{error:#}")),
   	}
   	}