Some improvements over init command

[mkniewallner]

Pull Request Check List

Resolves: #2554

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

Few improvements over init command:

• don't ask for description if already set through the CLI, similarly to what we do for other arguments
• don't ask for license if already set through the CLI and remove useless license validator, since it doesn't raise any error if set to empty or invalid (see the removed _validate_package and poetry-core helper that can confirm that)
• validate package definition by not accepting more than 2 whitespace separated parts, to validate the following things:
  • requests (valid, will search on PyPI API)
  • requests@^2.0 (valid)
  • requests 2.0 (valid)
  • requests 2.0 foo (invalid, and crashing today, as highlighted by Prevent badly-formatted dependencies from erroring poetry init #2554)
• reword package add prompt a bit to better indicate that a search through PyPI is performed when only specifying the name of the dependency

On a non-directly related note, I was thinking that it would be nice to be able to pass validators to the different options of the command, similarly to what we can already do in cleo for questions.

Right now, we are only able to apply the validators on interactive mode, but not when the user provides the options through the CLI.

Going even further and additionally to this, maybe adding a dedicated boolean in option could make it possible to automatically ask questions if an argument is not provided?
That way, the validator would be on option, and if the user doesn't provide it through the CLI, then they are asked to provide it, without having to create the question ourselves, and with the exact same validator as for option.

[github-actions]

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