disentangle filename conventions for renderers and variations

[macgyver]

per discussion in slack amphora-amp should use a different file name pattern to more harmoniously co-exist with component variations - it should be possible to specify a renderer and a variation as separate parts of a stylesheet filename.

currently AMP stylesheets are treated as component variations which makes them selectable options for other renderers, but we'd never want the AMP stylesheet to be used by the html renderer

[fraddski]

Agree that amphora-amp should use a different file name pattern for CSS so that AMP stylesheets aren't picked up by kiln as style variations