Qualify prebuilt_etc apex module name by relative paths

Currently, you can't have two prebuilt_etcs in one apex with the same
file name, because apexes emit make modules for all the files in the
apex, and the module name for the prebuilt_etc's make representation
is based on the base filename of the installed file. We can change it to
be qualified based on the full relative path of the outputfile so that
you don't hit the conflicts as much.

Fixes: 340207931
Test: Presubmits
Change-Id: I7836fd4661fcaafd91901eba7e0b89506946c3e2