Extract a helper function to resolve snippet files
and use it in the WebXML generator to output the
resolved snippet path. This can be used by PySide2
to fall back to a C++ snippet in case a matching
Python snippet is not present.

Task-number: PYSIDE-363
Change-Id: I182ae660227e97edf43e6dac906aeff9a5ae23f9
Reviewed-by: Martin Smith <martin.smith@qt.io>
Reviewed-by: Cristian Maureira-Fredes <cristian.maureira-fredes@qt.io>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>