plugins.dat usage in meson

plugins.dat has a custom_target rule in meson but there are a few issues:

1/ it does not depend on the list of vlc_module. If you update a module, it won't be regenerated by ninja.

2/ nothing depends on it, it's not actually generated

I gave this a try but depending on a custom_target doesn't seem to be usable yet in meson.

3/ it should not be generated/used in static builds