refactor(tvix/castore): add into_nodes(), implement consuming proto conv

Provide a into_nodes() function on a Directory, which consumes self and
returns owned PathComponent and Node.

Use it to provide a proper conversion from Directory to the proto
variant that doesn't clone.

There's no need for the one taking only &Directory, we don't use it
anywhere, and once someone needs that they might as well clone Directory
before converting it.

Update all other users of the `.nodes()` function to use `.into_nodes()`
where applicable, and avoid some more cloning there.

Change-Id: Id4577b9eb173c012e225337458898d3937112bcb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12218
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/castore/src/directoryservice/traverse.rs

@@ -36,7 +36,7 @@ where
 
                 // look for the component in the [Directory].
                 if let Some((_child_name, child_node)) = directory
-                    .nodes()
+                    .into_nodes()
                     .find(|(name, _node)| name.as_ref() == component)
                 {
                     // child node found, update prev_node to that and continue.