pipx: break `commands.py` into individual files

How would this feature be useful?

commands.py has grown to be quite large, so breaking it into file names corresponding to the command name will help reduce the barrier to entry for new devs in pipx’s codebase.

Describe the solution you’d like

Make a folder called commands and in that folder add files corresponding to each pipx command, such as commands/list_command.py, commands/install_command.py, etc.

Describe alternatives you’ve considered

Break commands.py into files in the same directory. This will make it harder to distinguish between libraries, classes and commands though, so I think putting commands in a commands dir will be the option of least astonishment.

Notes

• Created per suggestion in https://github.com/pipxproject/pipx/pull/246#pullrequestreview-304527855.
• Marking as good first issue because it’s purely a refactor. Existing unit tests should be sufficient.