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/nix-compat/src/derivation/errors.rs

@@ -15,6 +15,8 @@ pub enum DerivationError {
     InvalidOutputNameForFixed(String),
     #[error("unable to validate output {0}: {1}")]
     InvalidOutput(String, OutputError),
+    #[error("unable to validate output {0}: {1}")]
+    InvalidOutputDerivationPath(String, store_path::BuildStorePathError),
     // input derivation
     #[error("unable to parse input derivation path {0}: {1}")]
     InvalidInputDerivationPath(String, store_path::Error),