include referenced modules' items in selective import

test plan:
 0. have a course with a module containing an assignment, quiz,
    discussion topic, wiki page, file, and course-context external
    tool item
 1. have other assignments, quizzes, etc. in the course that are
    not part of the module
 2. export the course to a common cartridge
 3. import the course, selecting only the module
 4. verify that items referenced by the module are included
    (and those not referenced by the module aren't)

fixes CNVS-15226

Change-Id: I7fbed812a0b2edc3fad2bb75eb9aaab905923935
Reviewed-on: https://gerrit.instructure.com/40537
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>