Add --only-extra option to pip-compile

[atugushev]

Resolves #1959.
Refs #1957.

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• Included tests for the changes.
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• Verified one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
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[webknjaz]

I'm not sure if it is reasonable to support this. It's out of standard and encourages people to misuse Provides-Extra by exposing them to their end-users.

Extras are a public API for installing additional runtime dependencies together with the mandatory ones. All the installers, like pip, always include normal deps. Advertising this to people would cause confusion, especially, when used with pip install thing[extra] -c constraints.txt and --generate-hashes.

This PR should at least have a huge disclaimer explaining #1681 (comment) bundled in the docs and maybe the help string. And maybe don't call it --only-extra — having something more explicit like --only-extract-extra to signify that it's extracted as a standalone dependency source.

[webknjaz]

Oh, and maybe hold this until #1681 is merged? I don't want to keep introducing conflicts for the contributor all the time…

[atugushev]

I don't want to encourage bad practices, so I'll close this. If someone finds this feature sensible, please feel free to reopen the issue. Thanks for the feedback!

[webknjaz]

The dependency groups PEP is going to be the correct solution so it should be implemented instead. I think it'd okay to do this even before it's fully accepted.

See #2062.