DefaultCodegen: code-cleanup, propagate the openAPI instance

Created by: jmini

In DefaultCodegen we have some methods that work with the current openAPI instance, other that do not.

We have methods that use Map<String,Schema> (corresponding to openAPI.getComponents().getSchemas()) => this corresponds to allDefinitions in a OAS2.

Now that Components in OAS3 is more than just the schemas, all methods should work with openAPI.

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In particular:

The method public CodegenModel fromModel(String name, Schema schema) is really confusing. It is used a lot in the tests, but in reality public CodegenModel fromModel(String name, Schema schema, Map<String, Schema> allDefinitions) is used.

We should remove public CodegenModel fromModel(String name, Schema schema) or if we keep it, give a more explicit name.

And all Unit-Tests should use: public CodegenModel fromModel(String name, Schema schema, Map<String, Schema> allDefinitions) (or the adapted public CodegenModel fromModel(String name, Schema schema, OpenAPI openAPI) after the change described here)

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We will need to refer to Components more and more. For example if we want to solve #8 we need to look at the referenced responses and requestBodies.