Return correct library from `:pkgname` for github based recipe

If `:library` and `:feature` are not specified, `eval-after-load` will use `:pkgname` as its first argument.  For a github based recipe, the `:pkgname` has two parts: "username/reponame".  We need to remove the "username" part before passing `:pkgname` to `eval-after-load`.