Remove Extension.FullName() from interface

- this was something I did in the original implementation of this improved extension update notification logic
- discovering other parts of the extension manager code that were working with non-prefixed extension names motivated a different approach
- the extension manager code that requires the extension be prefixed has been enhanced to use the centralized ensurePrefixed() logic, making the need for this on the extension unnecessary

cmd/gen-docs/main.go

@@ -127,3 +127,7 @@ func (e *em) Create(_ string, _ extensions.ExtTemplateType) error {
 }
 
 func (e *em) EnableDryRunMode() {}
+
+func (e *em) UpdateDir(_ string) string {
+	return ""
+}