Fix include paths that match one file being included unconditionally

In aosp/1962181 an "optimization" was added where if an include
path with a variable only matches one file, that file would be
called directly instead of going through the whole _entry
variable.

This doesn't work when there is an inherit-product-if-exists call
that matches one file, but that file isn't the one that is specified
at runtime. In reality, the inherit-product-if-exists call should
see that the file doesn't exist and not include anything.

Fixes: 222341691
Test: go test
Change-Id: If8789de0013726137683078355161eecdec009b1