Preload should clear autoptimize cache

Each time Preload runs it should clear autoptimize plugin cache (if present), to avoid wasting space with duplicate minified CSS and JS files.

Start at the Preload implementation and trace how it creates minified CSS and JS files, then inspect how an installed autoptimize plugin exposes its cache. Done means each Preload run clears the autoptimize cache when it is present and does not leave duplicate minified files; add or update coverage for both plugin-present and plugin-absent cases.

Written by the indexing model from the issue text.