Fix issue when a path is '/' in a package

Summary:
when a path is '/' in defining a package, D1251 is generating
an extra '//'.

Test Plan: veryfied adding path '/', '/src' and '/src/' all worked.

Reviewers: nh, epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1266