If all packages in transaction are on hold, there's no
need to check for conflicts, shlibs, etc.
This makes `xbps-install -un` work faster on my system:
~0.450ms -> ~0.250ms
There's still room for more improvements :-)
`xbps_transaction_revdeps` will now try to add updates for packages to the
transaction if the dependency is not satisified anymore due to a package
install/update.
`xbps_transaction_prepare` will now check the return value of
`xbps_transaction_revdeps` and repeat the dependency resolution,
until `xbps_transaction_revdeps` returns 0, which means that it didn't
add any new packages to the transaction.
