Speak the platform vocabulary, not a parallel one #551

Epic: #446

NativePMAction{Manager PackageSource, Command PMCommand, Packages []string} restates what
platform.PackageManager and platform.PURL already model: PackageSource("apt") against
PURL.Type("deb"), and a four-value PMCommand enum against the manager's own Install / Remove / Upgrade.

Two vocabularies for one thing means a translation step between them, and the translation is where the
defects will live. An action should carry a PURL and reach a PackageManager.