PackageInstall akin to OciMount provider

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