This should ease migrating to a distinction between depot.third_party
and pkgs (as in nixpkgs) in the future.
Ref cl/2910, b/108.
Change-Id: I53a854071fddd7c0d0526cc4c5b16998202082c6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2913
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>