TIA: explicit class isnts: isExpansionInMainFile -> isExpansionInFileMatching

[hartogss]

Should we change from isExpansionInMainFile() to isExpansionInFileMatching?
We want explicit instantiations that are in user code, i.e. not in library code;
In order to answer that question, we have to study first where explicit class insts are placed in the AST.