refactor(nix-compat): Properly encapsulate store path construction

Before there was code scattered about (e.g. text hashing module and
derivation output computation) constructing store paths from low level
building blocks --- there was some duplication and it was easy to make
nonsense store paths.

Now, we have roughly the same "safe-ish" ways of constructing them as
C++ Nix, and only those are exposed:

- Make text hashed content-addressed store paths

- Make other content-addressed store paths

- Make input-addressed fixed output hashes

Change-Id: I122a3ee0802b4f45ae386306b95b698991be89c8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8411
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/cli/src/derivation.rs

@@ -412,16 +412,14 @@ mod derivation_builtins {
 
         // TODO: fail on derivation references (only "plain" is allowed here)
 
-        let path = nix_compat::store_path::build_store_path_from_references(
-            name.as_str(),
-            content.as_str(),
-            refs,
-        )
-        .map_err(|_e| {
-            nix_compat::derivation::DerivationError::InvalidOutputName(name.as_str().to_string())
-        })
-        .map_err(Error::InvalidDerivation)?
-        .to_absolute_path();
+        let path = nix_compat::store_path::build_text_path(name.as_str(), content.as_str(), refs)
+            .map_err(|_e| {
+                nix_compat::derivation::DerivationError::InvalidOutputName(
+                    name.as_str().to_string(),
+                )
+            })
+            .map_err(Error::InvalidDerivation)?
+            .to_absolute_path();
 
         state.borrow_mut().plain(&path);