[effects] Make scripted effects GHNS-able

Summary:
Currently, if one wants to install a scripted effect from the KDE Store,
the effect won't show up in the Desktop Effects KCM. The reason for that
is kpackagetool5 doesn't know where to install effects (they have to be
installed under ${DATA_DIR}/kwin/effects).

Another problem is that even if the scripted effect is installed in the
right directory (e.g. ~/.local/share/kwin/effects), it won't be listed in
the Desktop Effects KCM because it doesn't have a desktop file in
kservices5 dir. Please notice that the effect will be "visible" for KWin, i.e.
you can enable it by editing kwinrc.

This diff addresses those 2 problems by:

• Adding a PackageStructure plugin for effects (so they are installed under kwin/effects/);
• Using KPackage::PackageLoader to get list of scripted effect in the Desktop Effects KCM.

Test Plan:

• Installed an effect from the KDE Store, it appeared in the Desktop Effects KCM;
• Removed it.

Reviewers: KWin, mart, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: ngraham, davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D15372