Harmonize naming of Elasticsearch auto-configuration classes

[vpavic]

This PR harmonizes the naming of Elasticsearch auto-configuration components by including Elasticsearch in class names.

I've found the current naming to be very inconvenient to work with, since searching for Elasticsearch related auto-configuration components by entering ElasticsearchConfiguration or ElasticsearchProperties in IDEA's doesn't return some of vital components like RestClientAutoConfiguration and RestClientProperties - those two are especially harder to find this way since they're not in the same package as components that currently do have Elasticsearch in their name (org.springframework.boot.autoconfigure.elasticsearch.rest vs org.springframework.boot.autoconfigure.data.elasticsearch).

A good example of well-named (and therefore easily discoverable) auto-configuration components is org.springframework.boot.autoconfigure.session, where all components contain Session in their names.

Additionally, one further point of harmonization (that I didn't apply to this PR yet) could be to move components from org.springframework.boot.autoconfigure.elasticsearch.rest to simply org.springframework.boot.autoconfigure.elasticsearch - as reactive client components in org.springframework.boot.autoconfigure.data.elasticsearch aren't nested in rest subpackage.

[philwebb]

Dropping the rest package at the same time seems like a good idea, we can do that when we merge this one.

[vpavic]

Thanks for the feedback - I've applied the drop of rest package, the branch had conflicts from other things that landed to the master in the meantime so I was to update it anyway.