tpl/collections: Make it a package that stands on its own

See #3042

hugolib/hugo_sites_build.go

@@ -111,7 +111,7 @@ func (h *HugoSites) initRebuild(config *BuildCfg) error {
 		// This is for the non-renderable content pages (rarely used, I guess).
 		// We could maybe detect if this is really needed, but it should be
 		// pretty fast.
-		h.Tmpl.RebuildClone()
+		h.TemplateHandler().RebuildClone()
 	}
 
 	for _, s := range h.Sites {