refactor(nix-compat/store_path): make StorePath generic on S

Similar to how cl/12253 already did this for `Signature`, we apply the
same logic to `StorePath`.

`StorePathRef<'a>'` is now a `StorePath<&'a str>`, and there's less
redundant code for the two different implementation.

`.as_ref()` returns a `StorePathRef<'_>`, `.to_owned()` gives a
`StorePath<String>` (for now).

I briefly thought about only publicly exporting `StorePath<String>`
as `StorePath`, but the diff is not too large and this will make it
easier to gradually introduce more flexibility in which store paths to
accept.

Also, remove some silliness in `StorePath::from_absolute_path_full`,
which now doesn't allocate anymore.

Change-Id: Ife8843857a1a0a3a99177ca997649fd45b8198e6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12258
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/store/src/import.rs

@@ -8,7 +8,7 @@ use tvix_castore::{
 
 use nix_compat::{
     nixhash::{CAHash, NixHash},
-    store_path::{self, StorePath},
+    store_path::{self, StorePathRef},
 };
 
 use crate::{
@@ -115,7 +115,7 @@ pub async fn import_path_as_nar_ca<BS, DS, PS, NS, P>(
     directory_service: DS,
     path_info_service: PS,
     nar_calculation_service: NS,
-) -> Result<StorePath, std::io::Error>
+) -> Result<StorePathRef, std::io::Error>
 where
     P: AsRef<Path> + std::fmt::Debug,
     BS: BlobService + Clone,
@@ -161,7 +161,7 @@ where
     // callers don't really need it.
     let _path_info = path_info_service.as_ref().put(path_info).await?;
 
-    Ok(output_path.to_owned())
+    Ok(output_path)
 }
 
 #[cfg(test)]