Move support for sources into Target.

[jsirois]

There were several target subclasses re-implementing the boilerplate and
Target otherwise had intimiate knowledge of sources and an expectation
that the field existed for auxially methods.

This also allows for a general sources bag that can be used to express
file dependencies with no other expectation about the files' type or
purpose.

[jsirois]

This is part of the work for #4780

[stuhood]

I think the reason this is the way it is currently is that "not all Target subclasses accept the sources arg"... so I think that this change was cause all targets on http://www.pantsbuild.org/build_dictionary.html to accept that arg... but many of them would not do anything.

Would it be possible to continue to have this be opt-in, but make the opt-in much cheaper?

[jsirois]

Hmm, yeah. I was thinking there weren't many cases of this, but there are. I'll re-evaluate.

[jsirois]

I have a branch introducing a Files target that is ~Resources and which Resources can subclass to satisfy existing type tests that want to embed resource files in binaries. I'll move some of the other small changes here that are worthwhile to tiny PRs and close this out once the new Files PR is posted.

[jsirois]

Miscellany broken off into cpp: #4793 and go: #4794
I'll close this now and get the Files PR out after #4792 lands.