Move definition import implementation into the core

[michaelklishin]

With Prometheus and Grafana support in 3.8 some users would like to disable this plugin but it's not currently possible to export and import definitions without it.

The definition import modules are tied to this plugin for historical reasons. There is no reason why this feature cannot be available, say, via rabbitmqctl on a stock node. So let's make that possible.

The modules in this plugin would forward to the core ones, and the API endpoints will remain.
The goal is to keep everything backward compatible while making it possible to introduce a rabbitmqctl import_definitions or a similar command(s).