go-import-add の候補にシステムのパッケージが含まれなくなってしまったので、無理矢理回避

init.el

;; go-system-packagesの初期化はもう少しマシな実装がありそうな気がする

(defvar go-system-packages nil)
(use-package go-mode
  :ensure t
  :ensure go-eldoc
  :ensure flycheck-gometalinter
  :ensure godoctor
  :init
  (add-hook 'go-mode-hook 'go-eldoc-setup)
  (add-hook 'go-mode-hook 'flycheck-mode)
  (add-hook 'before-save-hook 'gofmt-before-save)
  :config
  (set-face-attribute 'eldoc-highlight-function-argument nil
                      :underline t :foreground "green"
                      :weight 'bold)
  (flycheck-gometalinter-setup)
  (setq flycheck-gometalinter-fast 1
        flycheck-gometalinter-test 1)
  (setq go-system-packages
        (let ((owd default-directory))
          (cd "~/")
          (let ((l (go-packages-go-list)))
            (cd owd)
            l)))
  :custom
  (go-packages-function
   (lambda ()
     (append go-system-packages
             (process-lines go-command "list" "-m" "-f" "{{.Path}}" "all")
             (process-lines go-command "list" "-e" "all"))))
  :bind
  ("M-." . godef-jump))