feat(tvix/store/pathinfosvc): add calculate_nar method

Putting this in the PathInfoService trait makes much more sense, we can
have direct control over where/how to cache the results in the
implementation.

This now requires each PathInfoService to hold pointers to BlobService
and DirectoryService.

Change-Id: I4faae780d43eae4beeb57bd5e190e6d1a5d3314e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8724
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/store/src/tests/utils.rs

@@ -12,6 +12,9 @@ pub fn gen_directory_service() -> impl DirectoryService + Send + Sync + Clone +
     MemoryDirectoryService::default()
 }
 
-pub fn gen_pathinfo_service() -> impl PathInfoService {
-    MemoryPathInfoService::default()
+pub fn gen_pathinfo_service<DS: DirectoryService + Clone>(
+    blob_service: Box<dyn BlobService>,
+    directory_service: DS,
+) -> impl PathInfoService {
+    MemoryPathInfoService::new(blob_service, directory_service)
 }