Bazel External Dependencies Overhaul

[Xudong Yang]

Hi all,

The new design for Bazel's external dependencies is ready for review. Given the expansive scope of the problem, the doc is rather massive (40+ pages), so we also wrote a tl;dr doc to explain the basic concepts.

Please have a read and let us know what you think! Your feedback would be much appreciated.

Thanks,

Xudong