Frontend plugins should not require being placed in the frontend
directory to be built successfully. Indeed they should only depend
on libobs and the obs-frontend-api and thus their source tree should
be able to exist anywhere (even standalone) and the plugin should still
compile successfully (just like any 3rd party plugin).
Thus moving those plugins into the main plugin directory ensures that
they don't require on any "special sauce" within the source tree to
compile.
