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

hugolib/testhelpers_test.go

@@ -536,6 +536,7 @@ func (s *sitesBuilder) changeEvents() []fsnotify.Event {
 }
 
 func (s *sitesBuilder) build(cfg BuildCfg, shouldFail bool) *sitesBuilder {
+	s.Helper()
 	defer func() {
 		s.changedFiles = nil
 	}()