feat(tvix/cli): add helper for populating derivation outputs

Adds a small helper function which uses a Nix value supplied to
`builtins.derivation{Strict}` to populate the `outputs` field of the
`Derivation` struct.

Change-Id: Iccc7a4f293b3d913140aed576a573a8992241e46
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7898
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/cli/src/main.rs

@@ -1,3 +1,4 @@
+mod derivation;
 mod errors;
 mod known_paths;
 mod nix_compat;