Refactor engines

[cnheitman]

This PR improves and simplifies the engines' packages:

• It merges each broker's add-ons with their respective engine package (for example, aflppbroker is now a module of pastisaflpp, called addon).
• It moves each engine's script inside their respective engine package (for example, pastis-aflpp is now a module of pastisaflpp, called __main__).
• It simplifies the directory structure (for example, engines/pastis-aflpp/pastisaflpp is now engines/pastisaflpp).
• It renames packages and module to make the naming consistent:
  • pastishf -> pastishonggfuzz,
  • pastisdse -> pastistritondse,
  • pastisdse.pastisdse -> pastisdse.driver,
  • pastisdse.pastisdse.PastisDSE -> pastisdse.driver.TritonDSEDriver, and
  • pastis-triton -> pastis-tritondse.