Reorganize `betty.service`

[bartfeenstra]

To do

• The goal is to flatten the module
• Move betty.service.factory to betty.factory
• Move betty.service.level to betty.service_level
• Merge betty.service.plugin.discovery into betty.plugin.discovery
• Move betty.service.plugin.service to betty.service.plugin or perhaps even betty.plugin.service

Blocked by

• Make plugin services declarative #4023