pkg/com: split into two packages

[progrium]

While refactoring Cmd.io on paper, and to better decouple config and registration, a plan/API became clear to split com into two packages: pkgcfg and objreg.

Personal notes for later:

objreg.Registered
objreg.New()
  Register
  Unregister
  Select
  SelectWith
  All
  AllWith

pkgcfg.Provider
pkgcfg.Setting
pkgcfg.SetProvider
pkgcfg.GetString
pkgcfg.GetInt
pkgcfg.GetBool
pkgcfg.CallerDistance
pkgcfg.Namespace