Allow optional packages in custom sets

[sputnick@…]

One feature that I like about Paludis' sets is that you can specify packages to be optionally in the set. By prefixing an atom with ?: in the set file, a package will not automatically be (re)installed except it's already in world. This allows one to only install and maintain parts of a set. There are also some more ways to specify hard and optional dependencies in sets.

Prime example are the sets provided by the genkdesvn overlay, which contain all packages belonging to a KDE module. By making all but the hard deps optional, you can choose the packages you really need by installing them explicitely (i.e. putting them in world), and still use the set for maintaining/upgrading these selected packages only.

[ferringb]

Patches desired, although before I'm willing to support this in world or /etc/portage/sets/*, you'll need to get portage folks to agree to support it.

For pkgcore specified sets (specifically, anything that isn't explicitly forced as portage configuration parsing), I have no major issues with this.

That said, I don't want to break compatibility w/ portage unless the feature is heavily needed and I know they're headed the same direction- please poke them to get confirmation of the latter.