refactor(tvix/castore): move *Node and Directory to crate root

*Node and Directory are types of the tvix-castore model, not the tvix
DirectoryService model. A DirectoryService only happens to send
Directories.

Move types into individual files in a nodes/ subdirectory, as it's
gotten too cluttered in a single file, and (re-)export all types from
the crate root.

This has the effect that we now cannot poke at private fields directly
from other files inside `crate::directoryservice` (as it's not all in
the same file anymore), but that's a good thing, it now forces us to go
through the proper accessors.

For the same reasons, we currently also need to introduce the `rename`
functions on each *Node directly.

A followup is gonna move the names out of the individual enum kinds, so
we can better represent "unnamed nodes".

Change-Id: Icdb34dcfe454c41c94f2396e8e99973d27db8418
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12199
Reviewed-by: yuka <yuka@yuka.dev>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/build/src/proto/mod.rs

@@ -1,9 +1,8 @@
 use std::path::{Path, PathBuf};
 
 use itertools::Itertools;
-use tvix_castore::directoryservice::NamedNode;
-use tvix_castore::directoryservice::Node;
 use tvix_castore::ValidateNodeError;
+use tvix_castore::{NamedNode, Node};
 
 mod grpc_buildservice_wrapper;