chore(tvix/store): move protos into separate mod.rs

This allows adding more stuff into this namespace, from different files.

Also move tests on proto-related code from src/tests to src/proto/tests.

Change-Id: I49e066fce90efbc18e16d68f94497b32ed5625c0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8091
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/store/src/tests/mod.rs

@@ -1,6 +1,3 @@
-mod directory;
-mod directory_nodes_iterator;
 mod directory_service;
 mod nar;
 mod path_info_service;
-mod pathinfo;