gstplugin: Handle static plugins in gst_plugin_load_by_name()

gstplugin: Handle static plugins in gst_plugin_load_by_name()

gst_plugin_load_by_name() assumed a plugin has a filename,
which isn't true for static plugins, leading to criticals.

If a plugin is already loaded, just return the loaded plugin,
which makes it work for static plugins as well as saving a
moment for already-loaded dynamic plugins.

Add locking in gst_plugin_is_loaded(), as a plugin may be
still being loaded in another thread.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3552>