hazel_custom_package_github should take multiple subdirectories

[judah]

Currently hazel_custom_package_github only uses a single directory. This makes the rules a little redundant when we have multiple of them, and also means we download the same repo multiple times (I think).

For example, we have three separate repositories for opentracing:
https://github.com/FormationAI/platform-core/blob/131037553cfebe6702769703c342fdff495da238/WORKSPACE#L352

Figure out an API and approach so that a single call to that macro can refer to multiple subdirectories (maybe mapping package names to paths).