Dedup logic for selecting system modules

Previously, the selection of system modules was duplicated, once for
source and once for prebuilts. This change dedups that by switching the
source code to use the same mechanism as the prebuilts which will ensure
consistent behavior in future.

Bug: 204189791
Test: m	nothing
Change-Id: Ia1729017ae332181c95f7b205dab87fb47d43fb8