Ignore license and similar dependencies of prebuilt_apex/apex_set

Previously, when determining which dependencies, direct or transitive,
of a prebuilt_apex/apex_set required APEX variants the code assumed
that all dependencies implemented ApexModule. While that is true for
the modules that can be explicitly mentioned in the exported...
properties it is not true for all of them. e.g. A
prebuilt_apex/apex_set can depend on license modules which do not
implement ApexModule.

This change simply ignores any module that does not implement
ApexModule.

Bug: 179354495
Test: m nothing
Change-Id: Iead6f9d1085d169335b88ceadcce2d8cc042254d