...to use short labels like `PLUGIN_PLAIN` and `PLUGIN_SSE2` rather than
`MODULE_NAME_IS_xxx_yyy[_zzz]`. this makes the code cleaner, easier to read,
and improves consistency. it also helps remove a source of plugin vs.
module confusion.

trying to recognise that something like `MODULE_NAME_IS_i420_yuy2` refers
to the 'plain' variant, compared to `MODULE_NAME_IS_i420_yuy2_sse2` for the
SSE2 variant, was problematic.

the `#else // defined(MODULE_NAME_IS_xxx_yyy_sse2)` instances have been
changed to `#elif defined(PLUGIN_SSE2)` in the i420 plugin.

the flag passed to the nv12 plugin was removed (rather than replaced with
`PLUGIN_PLAIN`) because it is unused.

the i420_rgb plugin used short `SSE2` type defines. these have been changed
to `PLUGIN_SSE2` style for consistency.