[LDCad] Rebrickable LDCad PBG export

[Orion Pobursky]

I found another bug.

Currently, if an LDraw number is listed for a part, the script takes that number unquestioningly. This results in the inclusion of a lot of alias parts which is the opposite of what I made this script to do. Instead it should check if the Rebrickable number is a valid LDraw part number and, if so, use that number.

This is now fixed.