Add ability to grab arbitrary dependencies

[mikehardy]

Once the proof of concept in #1 and #2 is done, make a pluggable (maybe) system that allows dependencies to be fetched from anywhere, either as nothing but an artifact (jar/aar) or following our bundle format (whatever that works out to)

[mikehardy]

Given new knowledge on #1 about publishing existing jars (not built from source) on jitpack I am less interested in this part - I would certainly accept PRs, but it appears making existing jars jitpack-available is easy, so just re-mapping coordinates is sufficient for almost all use cases, and certainly sufficient for mine (which is how I decide whether I will personally work on something or not)

1. add ability to verify mavenLocal is a valid repo for build script and for project build?
2. add ability to config a URL per lib instead of maven coordinates - that will be downloaded and installed in local repository for resolution can practice using the actual jCenter download URLs of the artifacts...do we need all the SHAs etc? if so that implies some sort of standard package and then something like github packages etc ?