* It no longer consists of a BPackageResolvableExpression and a
repository. Instead it can now either refer to a package directly or
consist of a search string.
* SolverPackageSpecifierList: Add AppendSpecifier() convenience
versions.
* Adjust LibsolvSolver and pkgman accordingly.
Not functional (or tested) yet. The libsolv setup for a somewhat
simplified installation case should be more or less complete, though.
The solution conversion to to-be-created Haiku data structures and the
handling of problems is still missing, though.