Restore exec dependencies during non-test system packaging

[cottsay]

The exec dependencies were dropped in a previous change to control the inclusion of test dependencies. The exec dependencies must be present to run tests, but because they aren't necessarily needed to build, they were dropped when the tests aren't going to run. However, the buildfarm's invalidation logic will remove downstream package which declare an exec_depend on a package, so the relationship must be maintained even though the invalidation isn't actually necessary.

Follow-up to #903

[cottsay]

Requires #995 to pass CI

[cottsay]

I think an open issue to make sure we continue to consider this sub-optimal status quo is worth having.

Filed as #1000