Skip registry upgrade only

[rikkimax]

It would be a good addition to add support for skipping the registry unless an explicit call to upgrade or if dub.selections.json becomes out of sync from the package descriptor.

Reference: https://forum.dlang.org/post/t4986c$1r40$1@digitalmars.com

[WebFreak001]

duplicate of #559

copying my response for closing:

DUB doesn't check for package updates when packages are already fetched & selected inside the dub.selections.json.

DUB only checks for upgrades:

1. with branch-based dependencies (~master) - they are discouraged/deprecated
2. for packages that don't have any selection yet in dub.selections.json or are not yet downloaded

Forcing no internet lookup (and causing a build fail for missing dependencies) can be forced with --nodeps