Architecture problem: all schemas in a single dependency

The `messages` library contains two disjoint schemas:

* `Gherkin` library (parser) output & test driver input (pickles)
* Pickle runner output (test driver output)

This causes a problem for the Gherkin library, because the version numbering of Messages applies to the collected schemas. When nothing changes with respect to Gherkin, it still may need a version bump because it needs to keep pace with Messages as it is consumed by the test runners -- which gets version bumps if the input/output of the test runner schema changes.

Proposal:

The proposal is to split the messages library into multiple libraries, each publishing exactly one schema.