Support "package" subcommand from Helm

[kkaempf]

Background: I'm using hypper as a full Helm replacement when working with Epinio and the latest release calls helm package which makes Hypper break.

Acceptance criteria: hypper is a full Helm replacement in an Epinio context

See also: epinio/epinio#556

[viccuad]

Initially I thought that it would be a good idea to add a hypper package, but the more I think about it, the more I would like to not wrap all of Helm with Hypper.

Hypper has backwards compat with Helm, and I would like to rely on that: everything should be working fine calling helm package, and installing the resulting charts with either Helm or Hypper (the latter of course brings more functionality).

For Helm commands that gain functionality when run in Hypper, I am all in though. Maybe we could add some Hypper-specific benefit to doing hypper package, such as checking for hypper repo consistency, but that sounds like a task for hypper repo index.

[mattfarina]

We should have the package command check the shared and optional dependencies for use of file:// for the repo and either throw and error or warning