feat(tools/emacs-pkgs): Add new builder for internal packages

Adds a new internal builder that makes it possible to override the
`emacsPackages` passed to our Emacs packages, which in turn makes it
possible to inject them into the emacsPackages fixpoint and use them
with features like Emacs native compilation.

Change-Id: I80dad57115c83cf5693ae6ba4e4cf3105d103d5e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3003
Tested-by: BuildkiteCI
Reviewed-by: adisbladis <adisbladis@gmail.com>
Reviewed-by: grfn <grfn@gws.fyi>

third_party/emacs/rcirc/default.nix

@@ -1,6 +1,6 @@
-{ pkgs, ... }:
+{ depot, ... }:
 
-pkgs.emacsPackages.trivialBuild rec {
+depot.tools.emacs-pkgs.buildEmacsPackage rec {
   pname = "rcirc";
   version = "1";
   src = ./rcirc.el;