Per mdempsky#32, a -source flag needs to be passed to this forked version of gocode in order for it to find references in foreign packages.
Since company-go in MELPA comes from this repo, it puts us in a strange position since nsf/gocode doesn't supports this flag. With Go's own Language Server Protocol server in development, the need for the gocode tool may "soon" be obviated. Have you considered splitting out the emacs packages into their own repos?