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/store/src/proto/tests/pathinfo.rs

@@ -66,7 +66,7 @@ fn validate_no_node(
     },
     Err(ValidatePathInfoError::InvalidNodeName(
         "invalid".to_string(),
-        store_path::Error::InvalidName("".to_string())
+        store_path::Error::InvalidName(store_path::NameError::InvalidName("".to_string()))
     ));
     "invalid node name"
 )]
@@ -111,7 +111,7 @@ fn validate_directory(
     },
     Err(ValidatePathInfoError::InvalidNodeName(
         "invalid".to_string(),
-        store_path::Error::InvalidName("".to_string())
+        store_path::Error::InvalidName(store_path::NameError::InvalidName("".to_string()))
     ));
     "invalid node name"
 )]
@@ -141,7 +141,7 @@ fn validate_file(t_file_node: proto::FileNode, t_result: Result<StorePath, Valid
     },
     Err(ValidatePathInfoError::InvalidNodeName(
         "invalid".to_string(),
-        store_path::Error::InvalidName("".to_string())
+        store_path::Error::InvalidName(store_path::NameError::InvalidName("".to_string()))
     ));
     "invalid node name"
 )]