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().

TODO

@@ -1,7 +1,6 @@
 libxbps:
  - transaction: avoid fetching the whole pkg when updating and only fetch
    modified files from target pkg.
- - Add support to handle 'shlib-requires' and 'shlib-provides'.
 
 xbps-create:
  - Move all configuration files to <prefix>/share/<pkgname>/conf/<cffile>.