Don't include PG INCLUDE columns as regular index columns

Don't include PG INCLUDE columns as regular index columns

Fixed issue where a "covering" index, e.g. those which have an  INCLUDE
clause, would be reflected including all the columns in INCLUDE clause as
regular columns.  A warning is now emitted if these additional columns are
detected indicating that they are currently ignored.  Note that full
support for "covering" indexes is part of :ticket:`4458`.  Pull request
courtesy Marat Sharafutdinov.

Fixes: #5205
Closes: #5206
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5206
Pull-request-sha: 512a3817bb21991142add2d192fa7ce9b285369d

Change-Id: I3196a2bf77dc5a6abd85b2fbf0ebff1b30d4fb00