Remove /boot/common for good

* Remove support for the "common" installation location from packagefs,
package kit, package daemon, package managers.
* Rename the B_COMMON_*_DIRECTORY constants referring to writable
directories to B_SYSTEM_*_DIRECTORY.
* Remove/adjust the use of various B_COMMON_*_DIRECTORY constants.
I'm sure some occurrence still remain. They can be adjusted when the
remaining B_COMMON_*_DIRECTORY constants are removed.

BSolverRepository: Add {Remove,Delete}Package()

BSolverRepository: add SetTo(const BRepositoryCache&)

BSolverRepository: fix gcc 4 build

BSolverRepository: Change priority from uint8 to int32

This allows us to specifies priorities below and above the user
definable range.

BSolverRepository: Add a change count

... so we can easily check whether the repository has changed since the
last time we used it.

BSolverRepository::AddPackage(): Add optional return parameter

... returning the added BSolverPackage object.

BSolverRepository: Add SetPriority()

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.

Remove /boot/common for good

* Remove support for the "common" installation location from packagefs,
package kit, package daemon, package managers.
* Rename the B_COMMON_*_DIRECTORY constants referring to writable
directories to B_SYSTEM_*_DIRECTORY.
* Remove/adjust the use of various B_COMMON_*_DIRECTORY constants.
I'm sure some occurrence still remain. They can be adjusted when the
remaining B_COMMON_*_DIRECTORY constants are removed.

BSolverRepository: Add {Remove,Delete}Package()

BSolverRepository: add SetTo(const BRepositoryCache&)

BSolverRepository: fix gcc 4 build

BSolverRepository: Change priority from uint8 to int32

This allows us to specifies priorities below and above the user
definable range.

BSolverRepository: Add a change count

... so we can easily check whether the repository has changed since the
last time we used it.

BSolverRepository::AddPackage(): Add optional return parameter

... returning the added BSolverPackage object.

BSolverRepository: Add SetPriority()

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.