Improve 'poetry add x y z' behavior when either x, y or z are already installed.

[mbello]

• I have searched the issues of this repo and believe that this is not a duplicate.
• I have searched the documentation and believe that my question is not covered.

Feature Request

The command poetry add x fails with an error if x is already installed. It is common to write scripts where one issues the full command e.g. 'apt-get install --y x y z' when you want to make sure x, y and z are installed without first checking whether any of those pieces are already present in the system. Similarly, I would like my script to not abort because poetry errors out when I ask it to add a package that is already installed.
Worse yet, when issuing poetry add x y z, if x is already installed, it will fail before installing y and z so it is not even possible to just ignore the error.

Possible solution 1: make poetry ignore installed packages, just issue a warning in those cases and then skip to the next package that needs to be added.

Possible solution 2: to avoid changing behavior in these cases (which I do not think would be a big deal), add a command switch to enable this desirable behavior

[seansfkelley]

FWIW, Yarn treats adding a package you already have the same as adding it anew: it sets the declared version range in package.json (i.e., pyproject.toml) to match at least the latest available version, then re-resolves and re-locks. I quite like this behavior, and often do poetry add by accident.

Edit: that said, apparently the docs already have an answer for this:

If you try to add a package that is already present, you will get an error. However, if you specify a constraint, like above, the dependency will be updated by using the specified constraint. If you want to get the latest version of an already present dependency you can use the special latest constraint:

[finswimmer]

This was implemented in #2886

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