Handle already existing targets of different name

In other words, if, in bp2build, module "foo" would generate "foo",
and "foo_two", and "foo_two" already exists in a build file,
bp2build should label "foo" as being unconvertible.

Fixes: 301321658
Fixes: 301312582
Bug: 285631638
Test: Unit tests
Test: Verified that `m bp2build` results in bit-for-bit identical
contents for out/soong/bp2build before and after this change.

Change-Id: Icbbdd69fce83579ec9b172d04b2bf1f294698f70