Extract logic to gather deps added for <apex>:<module> pairs

Trades having to traverse a module's direct dependencies multiple times
for reusable code. While at the minute the amount of duplicated code is
small (checking a tag and appending to a list) follow up changes will
modify the gatherApexModulePairDepsWithTag module to improve error
reporting greatly increasing the benefit of deduping.

The cost of traversing a module's direct dependencies is very small as
there will be only a very few dependencies.

Bug: 177892522
Test: m nothing
Change-Id: I16389102abd8038e1bfa52b63f4153bdf92ff553