Follow the cxx build for output file names

[ahayzen-kdab]

At the moment we use the file stem or the module ident or the cxx_file_stem depending on the situation in cxx-qt-build.

With CXX if the bridge is in src/folder/binding.rs they generate src/folder/binding.rs.h.

In this situation should we do the following	bridge type	proposed path	current path
#[cxx::bridge]	src/folder/binding.rs.h	binding.cxx.h
#[cxx_qt::bridge]	src/folder/binding.rs.qt.h	binding.cxxqt.h
#[cxx_qt::bridge(cxx_file_stem = "binding"]	binding.rs.qt.h	binding.cxxqt.h

Note that due missing support in macros we cannot find the path in the second case (as seen in #200 ) so ...

• [ ] For now disable having a cxx_qt::bridge without a cxx_file_stem until span source_file() API is stable