* Rename folder x-pack-core -> core
The jar remains 'x-pack-core-*.jar'
* Put group in top-level build.gradle instead of plugin/core/build.gradle
Original commit: elastic/x-pack-elasticsearch@b23452fa55
This creates a basic skeleton for the plugin split by adding folders and example
`build.gradle` files. It also includes a non-implemented `migrate-plugins.sh`
script that we can fill in at a later time.
Relates to elastic/x-pack-elasticsearch#2925
Original commit: elastic/x-pack-elasticsearch@2ab035d6b6
Michael Basnight
Lee Hinman