Provision incorrectly scans profiles for packages

Problem/Motivation

At the moment, Provision scans all profiles for modules/themes when it's been asked to scan for a single profile.
You can see this in the debug output of a platform verify, for example, the Standard and minimal core profiles for Drupal 8 contain no modules, yet scanning them will show modules being detected (if you have another install profile in the platform with modules).

I believe this is because of the fix in #2650290: Cannot find profiles beneath top-level directories which changed the scanning to be a hardcoded path to simply scanning all profiles. I think this was a bug in the code introduced there.

Proposed resolution

Fix the bug, so that if _provision_drupal_search_paths is being asked to look for modules within a specific profile, it searches within the profile folder only.

Remaining tasks

Review and test the patch.

User interface changes

None.

API changes

None.

Data model changes

None.