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 -- `. 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`.