Make `scan_plan_helper` internal

[kevinjqliu]

scan_plan_helper should be internal. it was introduced in d99936a#diff-23e8153e0fd497a9212215bd2067068f3b56fa071770c7ef326db3d3d03cee9bR1946

Previously, the local planning path was:

plan_files()
  -> _plan_files_local()
  -> self.scan_plan_helper()

With this refactor, local planning now goes through the extracted planner directly:

plan_files()
  -> _plan_files_local()
  -> self._manifest_planner.plan_files(...)

Since scan_plan_helper is an internal helper rather than a public extension point, this change seems fine. We should follow up and append an _ to the function name

Originally posted by @kevinjqliu in #3511 (comment)

[tanmayrauth]

@kevinjqliu I would like to work on this issue, can you please assign it to me?