Do not raise error if package is already in pyproject.toml on poetry add

[finswimmer]

At the moment poetry raises a ValueError when trying to add a package that is already in the pyproject.toml. This PR change the behaviour as follows:

• Do not raise ValueError if a dependency is added via poetry add and this dependency is already in pyproject.toml.
• Instead just print a warning and skip this package

Pull Request Check List

Resolves: #98

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

[sdispater]

I think from a purely UX standpoint, a good approach would be to display some guidance for user, for instance:

"package" is already present in the pyproject.toml file.

If you want to update it to the latest compatible version you can use `poetry update package`.
If you prefer to upgrade it to the latest available version you use `poetry add package@latest`.

[github-actions]

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