Drop the source_archive macro

[HoloRin]

This is quite a simple macro, but had the unfortunate side effect of introducing a dependency on rules_pkg. By dropping the macro, we can drop the dependency.

For reference, the macro was as follows:

def source_archive():
    source_tree(
        name = "source_tree",
        deps = [":erlang_app"],
    )

    pkg_tar(
        name = "source_archive",
        srcs = [":source_tree"],
        package_dir = "deps",
        strip_prefix = "source_tree",
        visibility = ["//visibility:public"],
    )