Add package-access-modifier proposal

[elsh]

No description provided.

[rjmccall]

Okay, here's my initial review as review manager. I'll take a look at the pitch thread tomorrow.

[rjmccall]

Notes from the review thread:

• Alternatives Considered should contrast this proposal with the idea of introducing submodules. Maybe we should talk that over offline at some point.
• Per Tony Allevato's comment, please discuss the possibility of enforcing that this isn't actually done outside of the package. (This would be possible if e.g. you had to pass the complete set of modules in the package to the compiler.)
• The discussion about dead-stripping and other optimization should be fleshed out.
• Please discuss the expected interaction of packages with library evolution. (e.g. this comment)

[benrimmington]

Would it be useful to link to SwiftPM's scoped identifier documentation?

[rjmccall]

Okay. This should be the final pass. Looking very close to ready!