Handle themes in the new file cache (for images, assets)

In the newly consolidated file cache implementation, we forgot that we also look in the theme(s) for assets (SCSS transformations etc.), which is not good for Netlify and the demo sites.

Fixes #5460

deps/deps.go

@@ -193,7 +193,7 @@ func New(cfg DepsCfg) (*Deps, error) {
 		return nil, err
 	}
 
-	fileCaches, err := filecache.NewCachesFromPaths(ps.Paths)
+	fileCaches, err := filecache.NewCaches(ps)
 	if err != nil {
 		return nil, errors.WithMessage(err, "failed to create file caches from configuration")
 	}