Restructure actuator packages to improve structure. The following
changes have been made:

 - Separate actuator and actuator auto-configuration into different
   modules.
 - Move endpoint code into `spring-boot-actuator`.
 - Move `Endpoint` implementations from a single package into
   technology specific packages.
 - Move `HealthIndicator` implementations from a single package into
   technology specific packages.
 - As much as possible attempt to mirror the `spring-boot` package
   structure and class naming in `spring-boot-actuator` and
   `spring-boot-actuator-autoconfigure`.
 - Move `DataSourceBuilder` and DataSource meta-data support from
   `spring-boot-actuator` to `spring-boot`.

Fixes gh-10261