Integrate FauxPackages

[nthykier]

Currently we have a helper script to generate and inject fake packages for various purposes. Britney should support these use-cases directly (which would also assist in fixing issue #2).

Known existing use cases:

• Avoid accidental removals (noremove.d - used by e.g. d-i)
  • Bonus for handling the "tasksel" case.
• Permit dependencies on certain missing packages (primarily for non-free packages that rely on third-party repositories)

Possibly new use-case:

• Ensure (co-)installability of certain package sets (https://bugs.debian.org/707219)