Port away from KPluginInfo

Corresponding functionality available mostly through KPluginMetaData. Additionally some bugs in script management were fixed in process. Requires KF 5.89, also some other at-that-point deprecated functionalities ported to later codepaths.

This required surprisingly small changes, although it affects very important functionalities (collection and service plugins in addition to scripts). So far, everything seems ok, but I'll keep test-using myself.