refactor(tvix/nix-compat): absorb //tvix/derivation

Put this in its src/derivation.

Change-Id: Ic047ab1c2da555a833ee454e10ef60c77537b617
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7967
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>

tvix/nix-compat/src/derivation/mod.rs

@@ -0,0 +1,15 @@
+mod derivation;
+mod errors;
+mod output;
+mod string_escape;
+mod validate;
+mod write;
+
+#[cfg(test)]
+mod tests;
+
+// Public API of the crate.
+
+pub use derivation::{path_with_references, Derivation};
+pub use errors::{DerivationError, OutputError};
+pub use output::{Hash, Output};