PXF client in gpdb uses pxf libraries from apache hawq repo. These pxf libraries will continue being developed in a new PXF repo greenplum-db/pxf and is in the process of getting open sourced in the next few days. The PXF extension and gpdb-pxf client code will continue to remain in gpdb repo.

The following changes are included in this PR:

Transition from the old PXF namespace org.apache.hawq.pxf to org.greenplum.pxf
(there is a separate PR in the PXF repo to address the package namespace refactor
greenplum-db/pxf#5)

Doc updates to reflect the new PXF repo and the new package namespace