top-level `arc-validate` command based on `validation_packages.yml`

[kMutagene]

A call to arc-validate (without subcommand) in an arc should:

• check if .arc/validation_packages.yml exists
  --> if false: do nothing (maybe print some diagnostic output)
  --> if true:
  • check if specification key exists in the YAML:
    • if true: if value is a valid specification module (we need to define a valid list somewhere): perform specification validation
  • check if validation_packages key exists in the YAML:
    • if true: install all packages from the list and perform validation against these packages