This doesn't make all these modules actually work with Qt6 yet, but it
prevents them from unconditionally pulling in Qt5 in a Qt6 build.

This is useful in order to even be able to pass a CMake run to the
point one can port, test and debug the rest.

While porting a manual configure_file invocation for kactivities'
.pc file to ecm_generate_pkgconfig_file I noticed that the URL keyword
was missing afterwards.

There is no path out of QtVersionOption.cmake that would let
QT_MAJOR_VERSION undefined, so this check is unnecessary. Worse, this
doesn't actually check for QT_MAJOR_VERSION being defined, but whether
a variable named after the content of the QT_MAJOR_VERSION variable is
defined. That is always false, so this effectively forces Qt5 and breaks
Qt6 support.

This reverts commit 8fd79163a7c60386fbc94bc14ca4a3b0b29d7227.

This fixes an infinite recursion of KDEInstallDirs including
itself when QT_MAJOR_VERSION is not defined.

and use only KSERVICESDIR in KDEInstallDirs6.

+ adjust docu