service module schemas should have flat directory structure

### What would you like to be added?

Service module schemas should have flat directory structure. For example, the following is preferred:

'''
import service as s

app := AppConfiguration {
workload: s.Service {
containers: s.Container {
readinessProbe: s.Probe {
}
}
}
}
'''

Instead of having to remember inner path of every schema:

'''
import service as s
import service.container as c
import service.container.probe as p

app := AppConfiguration {
workload: s.Service {
containers: c.Container {
readinessProbe: p.Probe {
}
}
}
}
'''

### Why is this needed?

Reduce the lines of code one has to write to use a module. Lower cognitive load.