refactor(tvix/glue): move builtins into separate directory

Change-Id: I25b7197458dbfbde8623545dc0a0286eb2744f10
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9911
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

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

@@ -1,3 +1,4 @@
+//! Contains [DerivationError], exported as [crate::builtins::DerivationError]
 use crate::store_path;
 use thiserror::Error;