Allow .unitypackage dependencies

[kibiz0r]

This would probably take a bit of annoying work, for not a whole lot of immediate gain since Unity packages don't really add any capabilities beyond what Paket could do for you anyway, but insist on having a slightly different format.

However, this would be the first necessary step to having smooth interop between Nuget/Paket and Unity's (fairly limited) ways of sharing code and resources.