Configure the go-packages-functions to use the gopkgs tool

This is a much faster option than the current native function.

layers/+lang/go/README.org

@@ -43,6 +43,7 @@ You will need =gocode=, =gogetdoc=, =godef= and =godoctor=:
   go get -u -v golang.org/x/tools/cmd/goimports
   go get -u -v github.com/zmb3/gogetdoc
   go get -u -v github.com/cweill/gotests/...
+  go get -u github.com/haya14busa/gopkgs/cmd/gopkgs
 #+END_SRC
 
 If you wish to use =gometalinter= set the value of =go-use-gometalinter= to t:

layers/+lang/go/funcs.el

@@ -71,3 +71,8 @@
    (format "go run %s"
            (shell-quote-argument (or (file-remote-p (buffer-file-name (buffer-base-buffer)) 'localname)
                                      (buffer-file-name (buffer-base-buffer)))))))
+(defun spacemacs/go-packages-gopkgs ()
+  "Return a list of all Go packages, using `gopkgs'."
+  (sort
+   (process-lines "gopkgs")
+   #'string<))

layers/+lang/go/packages.el

@@ -63,6 +63,7 @@
         "Set the tab width."
         (setq-local tab-width go-tab-width))
       (add-hook 'go-mode-hook 'spacemacs//go-set-tab-width))
+    (customize-set-variable 'go-packages-function 'spacemacs/go-packages-gopkgs)
     :config
     (progn
       (add-hook 'before-save-hook 'gofmt-before-save)