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.

So far we were defining e.g. KSERVICESDIR next to KDE_INSTALL_KSERVICES5DIR
here, rather than the intended KDE_INSTALL_KSERVICESDIR.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

SunOS and variants (Solaris, OpenSolaris, illumos, etc al.) do not need a separate library for inotify.

Might explain why the kcoreaddons CI isn't seeing ECM updates?

This enables forward-compatibility with KDEInstallDirs6.cmake

This was partly done already, but in the wrong order and not for all
versioned variables.