`mix hex.policy show` should list all package overrides

[ericmj]

Currently it only shows the count:

$ mix hex.policy show
Active policy: hex/production [public]

    hexpm:
      Cooldown:        14d
      Advisory rule:   block ≥ MEDIUM
      Retirement rule: SECURITY, DEPRECATED, RENAMED
      Overrides:       2

Effective cooldown: 14d (hex/production)

We should also remove the implicit show when running mix hex.policy. Running just mix hex.policy should be an error.