Decouple `openapi` and `json-schema` modules

[Baccata]

• Added a compiler-core module to hold logic common to the two modules that do not depend on either an openapi or a json-schema library.
• Privatised many constructs
• Reworked the package structure to facilitate sharing logic across openapi/json-schema without inter-dependencies
• Deduplicated as much logic as possible
• Renamed a few things

[daddykotex]

you're just missing a dependency on your new modules in the readme validator I believe