Make genrule export_include_dirs more consistent

Previously, genrule export_include_dirs always added ModuleDir to
exported include dirs when export_include_dirs is set but not when
export_include_dirs is not set. Now when export_include_dirs is set, we
also export the directory without the additional ModuleDir subdir.

Test: genrule go tests
Test: set export_include_dirs and test
Change-Id: I46e860b2c20c1a96bddd14367d7fa737d901994d