add support for source jars to scala_library

[jcoveney]

This is pulled out from #29

This adds support for source jars. This will be used and tested immediately by another PR

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[jcoveney]

closes #2

[johnynek]

what are we removing here?

[jcoveney]

good catch. fixed. this is from when I was moving around how the srcjar stuff was expanded

[johnynek]

👍 will merge when green.