Feature/cleanup and recursion

[mjheilmann]

This is an attempt to resolve the circular recursion case, in this PR:

1. Rework the state to store the descriptor once, so symbols reference a filename instead of holding a duplicate descriptor
2. Some state simplification, making some operations stricter
3. Add a function that shrinks the state by combining descriptors by package

The circular recursion fix in this PR is to blindly group descriptors by grpc package.

Pros:

• is a solution
• we do not need to detect circular relations of arbitrary length

Cons:

• we group everything, including externals. It misrepresents those a bit, but we have the same mapping problem as internal types
• we return descriptor responses for full packages, even if a user is only asking for a simple type. Fewer responses, but they could be larger than is optimal