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/castore/src/fs/fuse/tests.rs

@@ -15,10 +15,8 @@ use super::FuseDaemon;
 use crate::fs::{TvixStoreFs, XATTR_NAME_BLOB_DIGEST, XATTR_NAME_DIRECTORY_DIGEST};
 use crate::{
     blobservice::{BlobService, MemoryBlobService},
-    directoryservice::{
-        DirectoryNode, DirectoryService, FileNode, MemoryDirectoryService, Node, SymlinkNode,
-    },
-    fixtures,
+    directoryservice::{DirectoryService, MemoryDirectoryService},
+    fixtures, {DirectoryNode, FileNode, Node, SymlinkNode},
 };
 
 const BLOB_A_NAME: &str = "00000000000000000000000000000000-test";