refactor: fold `labgrid_manager` into `labgrid_config`

BREAKING CHANGE: `labgrid_manager` has been removed in favour of a
`manager` attribute in `labgrid_config`

As a result, `//labgrid/manager` has now become
`//labgrid/config:manager`. Also, `//labgrid/toolchain/manager:type` has
been removed.