kcms: Fix plugin id loading in desktop effects kcm

kwin installs metadata files for builtin effects so the kcm can find
them. With the recent KPluginMetaData changes, the Id field has been
removed, so the kcm will use "metadata" (the basename of metadata.json)
as the plugin id.

In order to fix plugin id resolution, let's install metadata with the
file name same as the plugin id.