Make qdoc work without module header

Since qdoc was ported to use clang, it requires to parse the module header to build up its AST for all C++ classes. While this works fine for Qt modules, it makes it hard to use for normal qt projects.

E.g. https://codereview.qt-project.org/#/c/256891/

To make neptune3 to pick up the autogenerated C++ files (they contain qdoc comments) an artificial module header needs to be created manually to pick up all files.

It would be great if qdoc would fall back to parse all headers in the headersdir when no module header was found.