Allow updating chart dependencies when packaging a chart

[seh]

At present when one runs helm package against a chart with entries in its requirements.yaml file but an empty charts directory (or even one partially populated), Helm fails with the following complaint:

Warning: is in requirements.yaml but not in the charts/ directory!

It would be helpful to be able to tell Helm to acquire any missing dependencies before packaging, since packaging can't proceed without doing so anyway. Per discussion in the "helm" Slack channel, consider adding a flag to the helm package command that would first run the equivalent behavior of helm dependency update.

Such eager updating can't be the new default behavior without surprising long-time users who are already used to the manual dependency updating requirement.

[technosophos]

The discussion in the slack channel basically suggested that we provide a helm package -u flag that would cause helm dep up to be run prior to the packaging logic.