In order to more easily integrate xpack once it moves into the
elasticsearch repo, references to the existing x-pack-elasticsearch need
to be reduced. This commit introduces a few helper "methods" available
to any project within xpack (through gradle project extension
properties, as closures). All refeerences to project paths now use these
helper methods, except for those pertaining to bwc, which will be
handled in a followup.
Original commit: elastic/x-pack-elasticsearch@850668744c
5a86450df7