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/lib.rs

@@ -1,2 +1,3 @@
+pub mod derivation;
 pub mod nixbase32;
 pub mod store_path;