Refactor completions to use cobras default completion command

[kangasta]

• [x] Depends on #139
• [ ] Depends on fix for https://github.com/spf13/cobra/issues/1740
• [ ] Remove replace directive from go.mod

Implements #134

[kangasta]

Closing this as #204 was merged in. We can handle possible cleanups in a new MR when cobra supports whitespace in bash completions.