Fix static plugins not working if - or . is used in pluginId

Those characters can't be part of the generated C++ class, but are used
in pluginIds. To avoid this, we only save the unmodified pluginId in the
internal list/target property.
The replacement to get a valid C++ class name is done only for the plugin
factory, which fixes this edgecase and makes the intention more clear.

While at it, the way we get the file path/pluginId for static plugins
was revamped. The StaticPluginMap contains the namespace + pluginId
already, meaning we could concatenate them at runtime.
This has the benefit of not having this information duplicated with the
custom moc-value.

To ensure if both compiles and works at runtime, the autotest was
extended.