fix: avoid unrelated schemaOverrides when building definition for single type

[lundibundi]

Please:

• Make your pull request atomic, fixing one issue at a time unless there are many relevant issues that cannot be decoupled.
• Provide a test case & update the documentation in the Readme.md

I want to use setSchemaOverride to override some types and then generate separate schema files per type with getSchemaForSymbol. Current implementation adds all overrides to each schema even if they are not used. This PR removes that behavior by default and adds an option includeAllOverrides to return it.
The includeAllOverrides is also enabled by default when generating schema for whole program.