completion: autogenerate

[casperdcl]

ye moste beautiful autogeneration of autocompletion

• auto-generate completion/dvc.bash based on dvc.cli use iterative/shtab to generate completions
  • add dvc completion [-s {bash,zsh}] [-f FILE] [{dir}] subcommand
  • add choices=(Required|Optional).DVC_FILE custom completion
  • add _dvc_compgen_dirs for e.g. output paths of get/import use choices=(Required|Optional).(FILE|DIR) custom completion
    • introduce consistency calling DVC-files targets and other files paths? not required Required to choose between _dvc_compgen_files and _dvc_compgen_DVCFiles
    • double-check the current changes
      • exclude some hidden/beta commands?
• make snap build its own completions
• add CI tests
• potentially drop _dvc_compgen_files for get/get-url/import shtab API offers separate completion of different args (i.e. don't complete url, but complete dirs for output path)
• potentially remove completion/dvc.bash altogether
• sort out completion/dvc.zsh too
• permanently puts tab completion scripts need more love #2985 to bed by loving really hard
• depends on alpha release shtab#2

much bash such python

follow-up tasks

• add pre-commit hook not needed as completions are dynamically generated now
• introduce consistency calling DVC-files targets, other files paths, and directories dirs
• introduce consistency around leading vs trailing whitespace when linebreaking strings
• update documentation completion: update installation instructions dvc.org#1447

[casperdcl]

ready for review now. The DeepSource test can be ignored (shell="bash"/"zsh" is mistakenly viewed as a security issue, likely due to confusion with Popen(shell=True))

[efiop]

This is great. Thank you @casperdcl !

[casperdcl]

Sure; feel free to add to #4070 with inevitable teething issues.