Support mods which conflict with themselves (yes, really)

[pjf]

A common use of provides is to indicate that some functionality (such as Life Support) is provided by the mod in question. Very often, that mod should be the only mod providing that functionality.

Rather than mods exhaustively listing other mods they conflict with, Debian policy allows for a package to conflict with itself, especially when that apparent conflict is done via provides.

This means we can have:

• TACLS provides LifeSupport conflicts with LifeSupport
• ECLSS provides LifeSupport conflicts with LifeSupport
• Snacks provides LifeSupport conflicts with LifeSupport

In this case, all these mods are mutually exclusive. If a new author comes along with a life-support mod, they merely need to declare that it provides LifeSupport (and conflicts with the same), and no other mod definitions need to be updated.

This is also important for the support of virtual packages, which may not provide any files themselves, but which provide relationships with other modules.

[pjf]

As of #244 we're doing this. Closing.