Image resource refactor

This commit pulls most of the image related logic into its own package, to make it easier to reason about and extend.

This is also a rewrite of the transformation logic used in Hugo Pipes, mostly to allow constructs like the one below:

    {{ ($myimg | fingerprint ).Width }}

Fixes #5903
Fixes #6234
Fixes #6266

resources/resource_cache.go

@@ -281,7 +281,7 @@ func (c *ResourceCache) DeletePartitions(partitions ...string) {
 
 	for k := range c.cache {
 		clear := false
-		for p, _ := range partitionsSet {
+		for p := range partitionsSet {
 			if strings.Contains(k, p) {
 				// There will be some false positive, but that's fine.
 				clear = true