Gen 2-5 Allow Combinations of Egg Moves

[pofehof]

Describe the problem
I noticed that if you have a Pokemon with Egg Moves that are only possible from different sets of fathers (Gen 3 Blastoise with Haze and Flail), PKHeX sees this as legal solely due to them being Egg Moves. From what I understand, starting in Gen 6, both parents can pass down Egg Moves, allowing combinations like this to occur.

Expected behavior
Egg Moves from different sets of fathers should be impossible in Gen 2-5 and seen as illegal.

[Lusamine]

This is a known issue but there isn't a good method to handle it right now, so it has not been implemented. What's needed is an efficient and general method of checking which combos are not possible, as opposed to a less good method like hard-coding every combination.