Improve and stabilize the protobuf validation

[mara-schulke]

cc @xfbs

We need to roll out more linting rules and restructure the validation implementation eventually.

@xfbs could you guide tom on this?

[xfbs]

Happy to — let's schedule something.

[xfbs]

Here's some rules that we have agreed we want.

Keep in mind that we don't want to build something that replaces a linter at this point — we only want to enforce specific rules that only buffers can enforce because they go beyond standard protocol buffer style.

• The protocol buffer package name should be the same as the buffrs package name, but with dashes (-) replaces with underscores (_)
• Buffrs packages can be of different types: api, lib, and impl. Buffrs packages which are lib packages should not have any service definitions. Only packages which are api packages should be allowed to have service definitions.
• File structure should be correct: any file which declares itself as being in mypackage.subpackage must live in the proto/subpackage/ folder. (might already be partially implemented?)

Other ideas:

• Ident casing: make sure all identifiers in a protocol buffer definition use a common casing, for example:
  • Message names are CamelCase
  • Field names are snake_case
  • Maybe this is not a good idea because a linter should be doing this?