tpl: Ensure site templates can override theme templates

The tplimpl package was misusing the TemplateLookupDescriptor.WorkingDir
field from the output package. By incorrectly setting it to the theme
directory instead of the site root, the user is unable to override theme
templates in some situations.

Fixes #3505

tpl/tplimpl/template.go

@@ -443,7 +443,6 @@ func (t *templateHandler) loadTemplates(absPath string, prefix string) {
 			)
 
 			if themeDir != "" && strings.HasPrefix(absPath, themeDir) {
-				workingDir = themeDir
 				layoutDir = "layouts"
 			}