Adopt AIP-0191 File and directory structure

[makahmad]

No description provided.

[toumorokoshi]

this one is pretty proto-specific: should we put this into a "protobuf" section?

the HTTP transcoding AEP would also fit in this category.

[rambleraptor]

I don't think this AEP is strictly necessary. There's a lot of Google-specific guidance in here (why do you need java_package_name but not go_package_name?) and none of it helps makes the APIs themselves any more intuitive / easy-to-generate.

[rofrankel]

@rambleraptor is going to drive discussion of whether we need this one.

[rambleraptor]

Closed in #186