`cljr-add-missing-libspec`: produce more friendly prompts

[vemv]

Closes https://github.com/clojure-emacs/clj-refactor.el/issues/430

Produce more friendly prompts in the following ways:

• Recommend to install a package
  • In the README and, conditionally, in the prompt itself
• Specify "Require" or "Import" depending on the completion candidates

For the use case "hit cljr-add-missing-libspec when the cursor is over List":

Before (no package installed, no completions in sight, says "require" for List which is confusing):

After: