refactor(tvix/castore): add PathComponent type for checked components

This encodes a verified component on the type level. Internally, it
contains a bytes::Bytes.

The castore Path/PathBuf component() and file_name() methods now
return this type, the old ones returning bytes were renamed to
component_bytes() and component_file_name() respectively.

We can drop the directory_reject_invalid_name test - it's not possible
anymore to pass an invalid name to Directories::add.
Invalid names in the Directory proto are still being tested to be
rejected in the validate_invalid_names tests.

Change-Id: Ide4d16415dfd50b7e2d7e0c36d42a3bbeeb9b6c5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12217
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI

tvix/castore/src/import/mod.rs

@@ -123,9 +123,8 @@ where
                 .path()
                 .file_name()
                 // If this is the root node, it will have an empty name.
-                .unwrap_or_default()
-                .to_owned()
-                .into();
+                .unwrap_or_else(|| "".try_into().unwrap())
+                .to_owned();
 
             // record node in parent directory, creating a new [Directory] if not there yet.
             directories