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_metadata_test.go

@@ -24,7 +24,7 @@ import (
 
 func TestAssignMetadata(t *testing.T) {
 	c := qt.New(t)
-	spec := newTestResourceSpec(c)
+	spec := newTestResourceSpec(specDescriptor{c: c})
 
 	var foo1, foo2, foo3, logo1, logo2, logo3 resource.Resource
 	var resources resource.Resources