To implement this, create a new Qt5BuildInternals package.
All child Qt modules like qtsvg should use
find_package(Qt5BuildInternals) or
find_package(Qt5 COMPONENTS BuildInternals) in the their
top level CMakeLists.txt.
This will make the qt_build_repo() macros available.
For qtbase we slightly cheat, and specify a CMAKE_PREFIX_PATH
pointing to the source folder that contains the BuildInternals
package.
For the other modules we actually use a configured and installed
package Config file.
This change moves variables that used to be written into the
QtCore Config file into the BuildInternals package. This way
things that are relevant only for building additional Qt modules
does not pollute the QtCore package.
Task-number: QTBUG-75580
Change-Id: I5479adff2f7903c9c2862d28c05c7f485ce3e4eb
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
