fix(tvix/nix-compat-derive): Get rid of external feature flag

The external feature flag was there because I couldn't find a way to
refer to crate and nix-compat with the same name so that the generated
code could be the same.

In essence `use nix_compat::nix_daemon:🇩🇪:NixDeserialize` is an error
when used inside nix_compat crate.

So my best fix was the external feature flag until I found the solution
used here which also removes the flag completely.

Change-Id: Ia3e89c6c350c3fb22ca87f974a39c21542aae152
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12376
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: Brian Olsen <me@griff.name>

tvix/nix-compat/src/lib.rs

@@ -1,3 +1,5 @@
+extern crate self as nix_compat;
+
 pub(crate) mod aterm;
 pub mod derivation;
 pub mod nar;