As discussed in [1], it's simpler to put the files in builddir/bin/, i.e.
no /plugins subdir, this makes it easier to run the unittest directly from
the CLI/debugger/IDE, without relying on ctest exporting the correct
QT_PLUGIN_PATH; instead QCoreApplication::libraryPaths() will pick up the
plugins in e.g. builddir/bin/kf5/kio/ automatically.

This will be followed by a change in KCoreAddons to remove the /plugins/ dir
from the path in the builddir.

[1] https://invent.kde.org/frameworks/extra-cmake-modules/-/merge_requests/213

It was missing the "/plugins" suffix; found while building KIO with Qt6,
where many unittests were failing with "couldn't create slave" for all slaves;
the kio slaves are in builddir/bin/plugins/kf5/kio, and KPluginMetaData
searches the directory "kf5/kio", and since it's a relative path, it assumes
it's in the "plugins" dir.

Keep the old behaviour for repos that put the plugins in builddir/bin/
directly without a "plugins" dir.

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.

This factors out large parts of the common code into separate modules,
and adds a backward compatibility wrapper.

The 6 variant drops some deprecated variables where possible, but otherwise
is the same as the 5 variant. It still lacks a replacement for the paths
depending on ECMQueryQMake though.

The description was a bit misleading and may have suggested that
ksvg2icons was required for @2x (Retina) icons to work.

When cross-compiling QT_INSTALL_BINS contains binaries for the target
system that can't be used during the build. Instead use QT_HOST_BINS
which contains the Qt build tools for the current system.

While debugging a failing test (KProcessTest in KCoreAddons), I noticed
that the Q_ASSERT() statements inside that test weren't being executed
because RelWithDebInfo builds default to passing -DQT_NO_DEBUG.
With this change the test I was debugging now asserts early instead of
failing a QCOMPARE later on.