Build dependency support for Addons

[yselkowitz]

Currently, Content Resolver does not support build dependencies for addon configs (e.g. ELN Extras). Therefore, build-only dependencies are not added, which means that both EPEL packages that are build-time only deps will not get added, and it cannot be seen if RHEL/CS ship the necessary subpackages or if CRB requests or -epel override packages are needed.

This is important for being able to use ELN Extras as a preparation for EPEL branching.

[tdawson]

This is done with this pull request:
#83

[tdawson]

I have created a separate issue, that will require a separate pull request, dealing with eln packages being seen in eln-extras.
#97