Improve OpenAPI logic

[ehmicky]

- Summary

This improves the OpenAPI logic:

• simplify the code
• do not directly mutate the OpenAPI definition exported by @netlify/open-api. We currently do it, which will cause bugs if the users are requiring @netlify/open-api in a different file.
• remove parameter.index (it is not used)

This also renames the following words to follow the official OpenAPI lingo:

• "method" renamed to "operation". This also avoids confusion with class methods and HTTP methods.
• "verb" renamed to "HTTP method".
• this was done only in the README and internal variables. The exported variables/properties are still called method and verb to avoid breaking changes.

- Description for the changelog

Improves the OpenAPI logic.

[coveralls]

Coverage decreased (-0.8%) to 65.69% when pulling 600b02a on feature/improve-open-api-logic into 3d40ef4 on master.