Remove buf.yaml and buf.lock as part of API by sending over structured module and dependency content

[bufdev]

This replaces #17 as #16 was closed.

This removes the sending of buf.yaml and buf.lock files over the wire, and consequently these being part of the structure of Modules as we know them, instead sending over the specific information we need on the backend in a structured manner.

This fully standardizes on the Node terminology. A Node is a pointer to some content, either on the request or response side. Within this API now, we have:

• FileNode: A pointer to file content.
• CreateCommitRequest.ModuleNode - A set of FileNodes and an associated ModuleRef that represents the content of a single Module..
• CreateCommitRequest.DepNode - A set of pointers to dependencies, which are just the commit_ids of the dependencies and their associated Digest (for verification).
• CreateCommitResponse.CommitNode - A set of pointers to files and then actual commit dependencies.

This renames GetFileNodes to GetCommitNodes.

To be honest, I don't love the Node naming, and I think it could use some work, but that's less important for the purpose of this PR. We can work on that. Ideas I've had:

• Info for new nested types, ie ModuleInfo, DepInfo, CommitInfo.
• Data for new nested types. In general, I think it's an anti-pattern to name a Protobuf message .*Data, as all Protobuf messages are data, and it doesn't pluralize well.

Open to other suggestions.