Moves first-party runtime modules into the "core" directory rather than
the "obs-plugins" directory to better differentiate them from legacy
plugins as well as existing 3rd-party plugins.
This change also requires the existing plugin directory to be explicitly
added as a possible module search path by application logic, as libobs's
internal logic will not add this path anymore and thus third-party
plugins would not be discovered anymore.
Co-authored-by: PatTheMav <PatTheMav@users.noreply.github.com>
