Improve speed of Click autocompletions

[matthewfeickert]

do we care if double-tabbing is slow? it can be slow for some applications, but as long as the normal single-tab to complete an argument like --outpu<TAB> is fast, then that's ok?

Originally posted by @kratsg in #2639 (comment)

In PR #2639 Click v8 native autocompletions were added, but they are quite slow. It would be good to figure out how the use of Click can be restructured to produce fast autocompletions for pyhf.