Fix finding dependents for deactivation

[fcollonval]

When trying to use it in jupyterlab-plugin-playground with JupyterLab 3.6.0-alpha.5. I realized the number of packages brings errors when linearizing the package dependency graph.

This filters the list of dependency relationship to the minimal graph related to the plugin to be deactivated.

[fcollonval]

@meeseeksdev please backport to 1.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout 1.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -x -m1 6e04a9a0abde723a1917327e99c80dae95eca5c9

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #490: Fix finding dependents for deactivation'

4. Push to a named branch:

git push YOURFORK 1.x:auto-backport-of-pr-490-on-1.x

5. Create a PR against branch 1.x, I would have named this PR:

"Backport PR #490 on branch 1.x (Fix finding dependents for deactivation)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.