Port kontact plugin loading to KPluginLoader

Summary:
Now that all kontact plugins are in plugins/kontact5 and embed JSON metadata,
we can load them directly without the indirection of .desktop files.
This depends on D28765 (in kcmutils) though, otherwise opening the configuration dialog crashes.

For zanshin this depends on D28847. Older versions of zanshin will no longer appear
in kontact once this commit lands (but new zanshin with old kontact will still work).

Test Plan: Starting kontact, using the configure dialog (and enabling/disabling components)

Reviewers: mlaurent, ervin

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D28848