[WIP] Deprecate path.plugins

[jordansissel]

I've intended to deprecate old-style plugin (older flag --pluginpath, --path.plugin) for years. Let's do it now :)

Details and discussion are in #6200

My intent primarily is to deprecate this feature, not remove it entirely, by doing the following:

• Remove references in documentation
• Warn on usage

Todo:

• Add documentation for how to move from old style plugins to new one (Add docs about moving from plugin.path to Gem based plugins #7677)

[ph]

I am +1 to remove that, the more features we add to plugins registry, like the universal plugins, makes this a bit harder to maintain.

Also, nothing prevent users to either install .gem or to define :path in their gemfile.

[ph]

Changes LGTM, waiting for travis to run the job.

[ph]

OOPS I havent read the WIP state on this PR, what do you think its missing @jordansissel ?

[jordansissel]

what I think is mainly missing is the docs I want to provide to users to upgrade -- specifically, a path for users to go from the old style to the new style plugins.

[suyograo]

Added an issue for the upgrade docs #7677. I will merge this one to 6.0 and 5.x.

[mergify]

This pull request does not have a backport label. Could you fix it @jordansissel? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label.

[mergify]

This pull request is now in conflicts. Could you fix it @jordansissel? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b deprecate-path-plugins upstream/deprecate-path-plugins
git merge upstream/main
git push upstream deprecate-path-plugins