Design: Macro-Aware Visibility

[brandjon]

This is a proposal for a new (backwards-compatible) visibility system that takes advantage of the upcoming Symbolic Macros feature. The new system would propagate and check visibility based on what symbolic macro we're currently running, rather than what package is being initialized. This enables better hiding of macro implementation details from callers.

[brandjon]

I've updated the proposal to reflect a decision to remove //visibility:exported from it, in favor of threading through a visibility attribute explicitly. I also added mention of how visibility attribute values are normalized. Most importantly, I replaced the example with a much more detailed one.