plugins.qmltypes incorrectly generated due to foreign types path

When building from qt-everywhere-src-5.15.1.tar.xz (and I assume the qt5.git), plugins.qmltypes files have incomplete definitions for Components when referencing types outside of the current module.

This happens because qmltyperegistrar is unable to resolve the foreign types files, resulting in (ignored) errors like these:

Cannot open foreign types file /usr/local/Qt-5.15.1/lib/metatypes/qt5gui_metatypes.json
Cannot open foreign types file /usr/local/Qt-5.15.1/lib/metatypes/qt5network_metatypes.json
Cannot open foreign types file /usr/local/Qt-5.15.1/lib/metatypes/qt5core_metatypes.json

 
(This example was from qtdeclarative/src/quick)

The root of this seems to be that the --foreign-types argument is being generated (in qmltypes.prf:48) using the install paths of these files and not the source tree paths.  When qt is being built in a "unified" build, those files are not yet installed.

One observed symptom of this issue is that users are unable to drag-reorder qml controls in the navigator view in QtCreator