As a Copybara user, I want to be able to operate on workflows via Bazel targets

#146 made it easier to load this workspace into another, which means that it’s easier for an external workspace to import the copybara tool and use it via bazel run <label> -- <args>. It’d be far nicer to be able to operate on workflows via bazel targets, something like:

copybara_migrate(
    name = "export_to_foo",
    workflow = ":copy.bara.sky",
    data = [
        "//some/copybara:library",
        ...
    ],
)

which we could then run via bazel run //path/to/target:export_to_foo.

[sudoforge]

@danielepolencic I never got around to cleaning up my personal rule lib and submitting it for use here. Thanks for the reminder; I’ll revisit this when I have the cycles to.

[danielepolencic]

@sudoforge is there a PR for the rules?