Upgrade to latest version of Cleo

[sdispater]

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code.

This PR upgrades the version of Cleo to the latest one. Cleo is now mostly a wrapper for CliKit which allows for a more flexible way to build a CLI.

Here are the main changes:

• New event system: This is the first step towards the implementation of a plugin system (see [Discussion] Poetry to provide a decent hook-based plugin system #693). It allows to hook into the commands resolution process to customize it. See https://github.com/sdispater/poetry/compare/develop...new-cleo?expand=1#diff-d43697d047b16e49f5630dc5839e8260R27 for an example.
• Sub commands: CliKit makes declaring sub commands easier. This effectively removes namespace commands and make them sub commands: poetry cache:clear becomes poetry cache clear, poetry debug:resolve becomes poetry debug resolve, etc...
• Easier command testing: CliKit and the new version of Cleo make it easier to test commands. Check the updated tests to see how.

Overall, this (big) PR will make it easier to build new features and plugins once a proper plugin system is implemented.

[github-actions]

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