Fixes the issue with oas2/oas3-schema rules in Studio.

Checklist

• Tests added / updated
• Docs added / updated

Does this PR introduce a breaking change?

• Yes
• No

Additional context

Why is it broken?
Some time ago we switched from dereferencing to bundling strategy when resolving $refs in rulesets during the Spectral offline assets generation.
This had one unintended side effect - as of that change all $refs are scoped relatively to the whole document.
This causes troubles, since in certain cases such has we pass a portion of the entire ruleset.
Having rewritten inline $refs inside of functionOptions is not really desired, because we always pass a given portion of the ruleset, therefore the $refs won't be resolvable, and eventually Ajv will refuse to validate the document.

  MissingRefError {
      message: "can't resolve reference #/rules/oas3-schema/then/functionOptions/schema/definitions/Info from id https://spec.openapis.org/oas/3.0/schema/2019-04-02#",
      missingRef: 'https://spec.openapis.org/oas/3.0/schema/2019-04-02#/rules/oas3-schema/then/functionOptions/schema/definitions/Info',
      missingSchema: 'https://spec.openapis.org/oas/3.0/schema/2019-04-02'
    }

The following PR makes sure we bundle only external $refs, while all inline $refs are kept untouched.