Package Kit/Launch Server: avoid/ignore use of requires keyword

In C++20 `requires` is a reserved keyword. GCC13 warns against its use. This
change removes the use of the keyword in our own code, and ignores it in the
external 'libsolv' code.

Change-Id: I3319d20c0a3fdad89630683cdff5ea3b17f04e99
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6647
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

kits/package: Break LibsolvSolver add-on out

* BSolver is implemented by solver add-ons
* We may want to (unlikely) leverage another
package solver in the future.
* We may want to (likely) implement a dummy
solver when libsolv is unavailable on new
architectures without bootstrap.
* This also makes solving missing libsolv a little
more graceful vs the "include "libsolv.h" not
found.

Change-Id: Iedd9d0f022fb743c4c7606bd33a4b6dbef0576f7
Reviewed-on: https://review.haiku-os.org/c/haiku/+/819
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>