revise plist methods in postflight mini-DSL

* accept index argument to `info_plist` in case of multiple `app`
  artifacts, defaulting to first
* return `Pathname` object from `info_plist`
* use `@command.run` instead of `system_command` which is intended
  as an external interface
* quoting
Merge pull request #7685 from rolandwalker/plist_postflight_review

review/revise plist methods in postflight mini-DSL