Define and Document what visibility really means

This is the culmination and attempted resolution of #8215. The commits have many more details about implementation details and the consequences of this refinement.

I'll point out specific locations which may be possible causes for alarm. In general, I have been very happy with how things have turned out. I'm a little sad that I couldn't remove privacy from resolve as much as I did, but I blame glob imports (although in theory even some of this can be mitigated as well).