Support for monorepos with multiple projects (#51)

[dtee]

There might be a better way to figure out the BUILD generator_location... but this works with multiple projects inside a mono repo.

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[ekuefler]

Thanks! Can you give an example of the kind of structure this allows for? Do the README lines at https://github.com/bazelbuild/rules_groovy/blame/master/README.md#L123-L124 need to be updated?