The regular upgrade check has been removed

Previously dub would regularly (once a day) check for possible package upgrades before building a packages.
This lead to unexpected build failures, e.g. when internet connectivity was down or dependency resolution failed, and caused unnecessary delays.

The build flag `--nodeps` now only suppresses resolution of missing dependencies.

The new upgrade flag `--dry-run` was added to explicitly check for upgradable packages without actually upgrading anything.