move QMAKE_PKGCONFIG_VARIABLES stuff out of qt_module_config.prf

move QMAKE_PKGCONFIG_VARIABLES stuff out of qt_module_config.prf

this only needs to be set in one module each - the one which provides
the relevant tool.

this is moderately source-incompatible, in that a package which queries
a given variable from the wrong library won't get the path it looks for
any more. as it's likely that everyone was using QtCore as a reference
anyway, this will only affect uic - which is in the new QtWidgets
library, to which people need to adjust anyway.

Change-Id: If05d3c33fda6cd12466e261391b825c59651d3e4
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>