Allow applets to have empty json metadata & use KPackage's metadata instead

This way we do not embed the json metadata in the plugins and can avoid the parsing of it at runtime.

The plugin id from KPackage metadata must correspond to the plugin filename. Because we often use the reverse domain notation, frameworks/kcoreaddons!202 (merged) is needed for correctly deriving the id.

In KF6 we can enforce this naming convention and thus get rid of the startup cache. This would further optimize the plugin loading, because we only need to load the libraries that we actually want to create plugins from.

Task: https://phabricator.kde.org/T14757

Draft for now, because I need to finish the work in plasma-workspace, plasma-desktop & kdeplasma-addons