Services: Make services of services also be persistent

[aomarks]

Previously, only a directly invoked service persisted across watch iterations. But actually, if one of those services itself depends on a service, then that service should persist too.

So this PR renames "isDirectlyInvoked" to "isPersistent", and expands its definition to include services of services. We call non-persistent services "ephemeral".

Also allows start to be called multiple times (an obvious case to handle, just hadn't hit it in the tests so far).

Example:

                       start
                   (no-command)
                    /        \
                   ▼          ▼
             serve:api      serve:static
  (persistent service)      (persistent service)
          |                          |
          ▼                          ▼
      serve:db                   build:assets
  (persistent service)            (standard)
                                     |
                                     ▼
                             serve:playwright
                            (ephemeral service)

Part of #33