Support BOM (bill of materials) poms

[cgruber]

BOMs provide pure-metadata references which collect together a package of artifacts. They can be used to provide a single dep which "expands" into a whole set of fixed versioned artifacts. Bazel maven repository should support the use of these.

The basic behavior should be to do one of two things.

1. Substitute the single bom with its contents
   • This is more what maven had in mind, since the bom sort of doesn't exist
   • Much more complex on kramer, since it adds a second layer of resolution.
2. Create a no-artifact intermediate dep and treat the bom's deps as that target's deps
   • Adds an indirection - an empty target that is a "pass-through"
   • More bazely - the bom is real, but only metadata