Recursive handling of optional dependencies

[dragonchaser]

When we deal with the optional dependencies, atm the user is being asked which ones to install.
There are scenarios where there could be optional 2nd, 3rd... n-th level optional dependencies.
Currently those are ignored. My proposal would be to solve the tree several times, workflow:

1. Have the user select optional dependencies for the current package
2. Resolve for this tree
3. If the shared- or optional-dependencies have optional dependencies of their own GOTO 1 and repeat until the whole tree is resolved, similar on what zypper does when running zypper dup

[viccuad]

Another option instead of recursively finding optional dependencies:

1. Solve once, tagging the packages that are pulled in as optional with an p.IsOnlyOptionalToSomeOtherPackage = true.
2. List those optional packages and ask the user on which ones they would like to install.
3. Promote the ones wanted to normal package dependencies.
4. Solve again.

Resolving recursively means that we would need to detect loops (which can be bigger than the amount of packages), prune, etc, and basically we would be reimplementing the sat solver. Hence, better if we don't do it.