Add BSolverPackageSpecifierList::AppendSpecifiers()

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.

Flesh out the package kit solver API quite a bit more

* Reorganize things a bit:
- BSolver is now an abstract base class.
- A libsolv based implementation, LibsolvSolver, lives in a new
add-on, which is loaded lazily.
- Get rid of libpackage_solver. Save for LibsolvSolver everything
is moved to libpackage.
- This is a nicer solution for the cyclic dependency caused by
libsolv (libsolvext to be precise) using the package kit for
reading repositories and package files.
* Add a solver result data structure and and an accessor the solver.
* Add problem reporting support to the solver. There aren't data
structures for the problem solutions yet and support for selecting
solutions and re-solving is missing as well.

Beginnings of the PackageKit dependency solver

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.

Add BSolverPackageSpecifierList::AppendSpecifiers()

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.

Flesh out the package kit solver API quite a bit more

* Reorganize things a bit:
- BSolver is now an abstract base class.
- A libsolv based implementation, LibsolvSolver, lives in a new
add-on, which is loaded lazily.
- Get rid of libpackage_solver. Save for LibsolvSolver everything
is moved to libpackage.
- This is a nicer solution for the cyclic dependency caused by
libsolv (libsolvext to be precise) using the package kit for
reading repositories and package files.
* Add a solver result data structure and and an accessor the solver.
* Add problem reporting support to the solver. There aren't data
structures for the problem solutions yet and support for selecting
solutions and re-solving is missing as well.

Beginnings of the PackageKit dependency solver

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.