hab pkg install doesn't write/run install hooks to /hab/svc if /hab/pkgs already populated

[themightychris]

In CI and test environments, it can be helpful to persist /hab/pkgs but not /hab/svc, which seems to be in the spirit of Habitat's architecture -- /hab/pkgs is immutable and /hab/svc is instance state

However, there does not seem to be any way to get hab pkg install to regenerate install hooks under /hab/svc if a package already exists under /hab/pkgs but /hab/svc is starting fresh. The package must be fully uninstalled and then reinstalled to get the install hooks to exist and be run. In this way, /hab/pkgs is being used to indicate the state of an instance which feels like a violation of Habitat's guarantees.

Instead of existence of /hab/pkgs content for a package being the signal for whether a package has already been installed and needs its install hook ran, perhaps the rendered install hook under /hab/svc should be used instead. The expected behavior would be that every time hab pkg install is run, if the rendered install hook doesn't exist under /hab/svc it gets rendered and executed.

[stale]

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