feat(tvix/cli): track known plain paths in NixCompatIO

When adding things to a C++ Nix store, ensure that the path is tracked
in the tracker.

Since the mechanism for propagating the tracker instance isn't
finalised yet, I've opted to take an Rc<RefCell> parameter for it. How
exactly that ends up there is going to become clear in the next
commits, but for now it's just instantiated in main with
Default::default.

Change-Id: I90f0b44f2d4f292dedc98ff1aa39041d279b61fd
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7833
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/cli/src/known_paths.rs

@@ -29,6 +29,7 @@ pub enum PathType {
     Plain,
 }
 
+#[derive(Debug, Default)]
 pub struct KnownPaths {
     /// All known paths, and their associated [`PathType`].
     paths: HashMap<String, PathType>,