Fix loading of external extractors and writers
In 5bbdfc47, the external extractors were mistakenly ported to KPluginMetaData::findPlugins.
However, the logic was broken before, because the dir entries must not be plugins, but normal files. This was introduced in 73da9a53.
Considering that we tell QDir to only list dirs, the check if we have a plugin or not doesn't make sense anyways.