Ensure no unnecessary class parsing is performed

[vmutafov]

Currently there is a step in the SBG which parses all JAR dependencies and collects Java interfaces in them in order to be provided for the js parser. Another step after the js parser parses all JARs too. Those can be unified without much effort. Also, other build time generators perform the same JAR parsing. Combining the generators or at least performing the JAR parsing just once could improve the initial build time.