Added support to use pkg shlib-{provides,requires} objects.

This effectively checks that all pkgs in a transaction to be installed
or updated, have not unmatched shared libraries, which might happen with
pkgs that had major soname bumps.

This implementation slow downs xbps_transaction_prepare() 2x, due to
the use of xbps_pkgdb_get_pkg_metadata(). A new pkgdb format is required
that adds the pkg shlib-provides objects to pkgdb, avoiding the need
to use xbps_pkgdb_get_pkg_metadata().