implement git_subdir dependency specification

[tomjnixon]

This is supported by rebar3, and mostly useful to use one app from an umbrella project:

https://www.rebar3.org/docs/configuration/dependencies/

This uses sourceRoot, which might not be the best approach, though it seems to be the common way in nixpkgs. An alternative would be something like:

      src = builtins.path {
        path = (fetchFromGitHub {...}) + "/sub/dir";
      };

Which is a bit more complicated, but nice because it doesn't cause a rebuild when source files outside of the specified sub directory change.

Other changes:

• switched to using the pname argument for rebar3Relx, which is requited since nixos-21.11. I considered trying to make it work with either argument style, but that doesn't seem necessary for such an old change.
• updated nixpkgs in tests, and pinned a dependency (which was causing the tests to fail with the old nixpkgs version).