PackageInstall akin to OciMount provider

[srak289]

Suggested by Klee:
Some declarative spec for packages required by a MachineModel.

Something perhaps like:

add_provider(PackageRequirement(package='exim', requires_running=True))

Where PackageRequirement could be picked up by a mixin class such as
AptManagedMixin or RpmManagedMixin .. for example.

This would shift the current paradigm from setup_tasks manually running apt update and install to a mixin class responsible for gathering requirements and installing them as one (or possibly two if requires_running=True) step(s).

Perhaps eclipses #111