refactor(tvix/glue/register_in_path_info_service): return only PathInfo

The store path is already contained in the PathInfo, and the ca bits is
already passed into the function, so known to the caller - there's no
need to duplicate this.

We can also avoid having two separate block_on in our import builtin -
we already know the content hash before constructing, as we pass it in
via ca_hash.

There's still some room to unclutter some more of the code around
importing - we still do NAR calculation twice in some cases, and some of
the code might be share-able from other places producing PathInfo too.
Log a TODO for this cleanup.

Change-Id: I6a5fc427d15bc9293a396310143c7694dd2996c0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12592
Reviewed-by: Marijan Petričević <marijan.petricevic94@gmail.com>
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/docs/src/TODO.md

@@ -77,6 +77,20 @@ correctness:
    "some amount of outgoing bytes" in memory.
    This is somewhat blocked until the {Chunk/Blob}Service split is done, as then
    prefetching would only be a matter of adding it into the one `BlobReader`.
+ - The import builtins (`builtins.path` and `builtins.filterSource`) use(d) some
+   helper functions in TvixStoreIO that deals with constructing the `PathInfo`
+   structs and inserting them, but some of the abstractions where probably done
+   at the wrong layer:
+    - Some ways of importing calculate the NAR representation twice
+    - The code isn't very usable from other consumers that also create structs
+      `PathInfo`.
+    - `node_to_path_info` is ony called by `register_in_path_info_service` (due
+      to this marked as private for now).
+   Instead of fighting these abstractions, maybe it's best to explicitly add the
+   code back to the two builtins, remove redundant calls and calculations. A
+   later phase could then see how/if some of this can be reasonably factored out in
+   a way it's also usable for other places having a node and wanting to persist
+   it (if at all).
 
 ### Error cleanup
  - Currently, all services use tvix_castore::Error, which only has two kinds