pkgman install: Simplify transaction creation
<Archivable.h>: Fix conflict with BPackageKit::BPrivate

... when "using BPackageKit::BPrivate".
pkgman: Refactoring -> PackageManager

Move common and reusable functionality from "search" and "install" to
new PackageManager class.
BSolver/LibsolvSolver: Add Uninstall()

Also fix incorrect check in LibsolvSolver::GetResult().
pkgman install: Fix typo in usage text
pkgman: Add "uninstall" command