Root package contents are not displayed

Due to a logical error the contents of a root package were never shown.
This commit fixes that error and corrects a logical problem where
namespaces were still shown despite that they had no contents.

Fixes https://github.com/phpDocumentor/phpDocumentor2/issues/1160
Add a test for root constants
Merge branch 'develop' of github.com:phpDocumentor/phpDocumentor2 into develop