Moved pillar scheme logic to contents param.

Per discussion in issue #6508 applying a `pillar://` scheme to the
`source` parameter will be confusing because there is no parity with
other locations that use `source` and the most useful location for this
is the `file.managed` state.  In order to alleviate the problem, apply
the logic to the `contents` parameter, which is only supported in
`file.managed`.