getEClassForClass(): Also cache list of registered EPackages

* Obtaining EPackages from the registry is expensive.
* This speeds every (n > 1)th invocation of the method, regardless
  of the class for which the EClass is required.
* The cache that directly memorizes previously computed results is
  kept.

Issue-Ref: 3675
Issue-Url: https://af3-developer.fortiss.org/issues/3675



Signed-off-by: Simon Barner <barner@fortiss.org>