Cache filename (filemtime part) handling for imported/resolved CSS files

The filename of a CSS resource bundle only contains a key for the freshness of the outer files (most recent filemtime), not the @imports.

So, if you change e.g. basic.css, which gets imported only, the cache does a) not get invalidated (which is expected), but after clearing the cache, the generated filename would be the same (and would not contain the most recent filemtime of all files being read in).

Possible solution: changing the mtime of the "main file" (where the @imports get resolved from) to the one from the newest imported file?! This keeps regular lookups cheap (so that they do not have to follow the @imports to get the most recent mtime).