Warn when files of a QML module are outside the directory

This was born out of the QTBUG-128649 . To get better understanding of the context, it might be helpful to take a look at it first.

In the initial task there was a weird issue with paths produced or managed by the qmlcachegen, triggered by adding a file to the qml module which was located outside of the qml module dir or its subdirs.

However it seems that the issue was not reproduced on linux, moreover 2 mitigation actions were proposed:

1. Document that files logically belonging to the qml module should be inside the directory of the module or its subdirs. (which was done as part of QTBUG-128649)
2. Add a warning, pointing to that piece of documentation. <- This task

A good starting point might be https://codereview.qt-project.org/c/qt/qtdeclarative/+/626200 and the relevant comments on that patch