lisa.energy_meter: Use SimpleMultiSrcConf for conf classes
Mirror what is done in TargetConf. Avoid having an extra "conf" level in the expected YAML format, which is useless in this kind of configuration.
GitLab Maintenance scheduled for Thursday, 18th December 2025, from 7:00 AM to 10:00 AM BST. GitLab will be in read-only mode and experience short periods of complete downtime (up to 20 minutes) during this window.
Mirror what is done in TargetConf. Avoid having an extra "conf" level in the expected YAML format, which is useless in this kind of configuration.