m <module_name> builds <module_name>.<apex_name>

When a module is not available for platform (i.e.
//apex_available:platform is missing in the apex_available property), m
<module_name> previously just didn't work because there is no platform
variant of the module.

This change fixes the behavior; regardless of whether the platform
variant is available or not, m <module_name> builds all the apex
variants of the module along with the platform variant if it exists.

Bug: 147728094
Test: m conscrypt
Change-Id: Iedd3fa6fc0ed779c5f7c5d65f23d86f799ac0cbe