This change is the x-plugins side of elastic/elasticsearchelastic/elasticsearch#14952. It
now requires x-plugins to be checked out as a sibling of elasticsearch
called extra-plugin-x-plugins.
Original commit: elastic/x-pack-elasticsearch@f69b167407
This change removes the project attachment support, and instead relies
on x-plugins being checked out under extra-plugins/x-plugins within an
elasticsearch checkout. The only real change, other than removing
unnecessary code, was to rename the license/core project because gradle
project substitution has a bug which causes it to try and use ES core as
the substitution. (Unfortunately this is not reproduceable with a simple
example, so I have not yet filed an issue with gradle).
Original commit: elastic/x-pack-elasticsearch@fa315ffcb5
Ryan Ernst