ensure no duplication of build files

if you have two bazelize elements in a project, and one is a none-rule then they would produce the same BUILD content if they lie in the same dependency tree. If the buildfile-dirs are not equal then they will produce subpackages with the same targets

for example the root BUILD specifies a library with sources at /usr but another is now at /usr/BUILD which makes /usr a subpackage. That will break source binding for targets at root

however, we need to separate and allow legitimate subpackages