Rework SolverPackageSpecifier

* 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.
pkgman: C++-ify the command handling

There's now a Command class that must be derived and registered with
a CommandManager, all simplified by a REGISTER_COMMAND macro. That gets
rid of the print_command_usage_and_exit() function copy for every
command, moves the short usage texts to the command implementations,
and avoids any repetition of the command name. When implementing a new
command only a new source file needs to be created, nothing else needs
to be touched.