Protocols with default methods in extension fail to compile

If you have a protocol which has a default method implemented in an extension, the generated code will fail to compile because the Java compiler will complain about the extracted interface "is not abstract and does not override abstract method".

Consider the following

protocol MyProtocol {}

extension MyProtocol {
  public func myDefaultMethod() {}
}

will fail with:

error: MyProtocol is not abstract and does not override abstract method myDefaultMethod() in MyProtocol

Instead we should not generate any methods, variables and initializers defined in extensions as part of the extracted interface. But we could generate them as default methods instead. This means we need to change the analysis phase to know what decls are defined in an extension and which come from the protocol requirements themselves.

Start with the analysis phase and extracted interface generation described in the issue, then reproduce the protocol and extension example. Trace how protocol requirements are distinguished from declarations in extensions; done means the example compiles without treating the extension method as an abstract interface requirement, with extension declarations represented according to the intended default-method behavior.