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/tests/mod.rs

@@ -1,7 +1,7 @@
 use crate::derivation::output::Output;
 use crate::derivation::Derivation;
 use crate::nixhash::NixHash;
-use crate::store_path::{build_store_path_from_references, StorePath};
+use crate::store_path::StorePath;
 use std::collections::BTreeSet;
 use std::fs::File;
 use std::io::Read;