Add option to export only dev-dependencies

[vchrombie]

This PR adds the support, for exporting only the dev-dependencies, to the export plugin.

Related to python-poetry/poetry#1441

Opening a PR here too, as the export function will be soon moved as a plugin.

Extension to python-poetry/poetry#4283

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• Updated documentation for changed code.

[vchrombie]

Hi @sdispater, I think the dependency groups feature would make it possible to solve this issue.

--only: The only dependency groups to include when exporting.

I understand poetry export --only dev -f requirements.txt --output requirements.txt something along this command would make it possible to export only the dev-dependencies (assuming the dev-dependencies are grouped under dev).

I know it might take some time to publish new release to have all these features, but could you confirm if my assumption is right? If yes, I think we can go ahead and close this PR too.

Thanks.

Best,
Venu

[vchrombie]

Closing the PR in favour of #3 (comment)