stackrc now requires GITREPO, GITBRANCH, GITDIR and has been
dependent on functions for a while (is_package_installed). Ensure
we source the required functions file when stackrc is loaded. Avoids
unexpected issues in grenade where they may or may not have been loaded
depending on the configuration.
Closes-bug: #1377274
Change-Id: I5027cfad07af0de7ff39f424601d6f7ec5dcadae