chroma: tidy plugin simd variant flags

...to use short labels like PLAIN_PLUGIN and SSE2_PLUGIN 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 i420_rgb plugin used short MMX type defines. these have been changed to MMX_PLUGIN style for consistency.

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also, added extra commit, in relation to discussion below, addressing similar flags in two other plugins.