Don't redownload jquery when installing from an sdist

[AWhetter]

Closes #14585

PR Summary

The source for jquery-ui is already made available as part of the sdist. This change avoids downloading jquery-ui at all if installing from the sdist and jquery-ui can be found locally.

The use case I have is we don't have access to the internet. We access matplotlib through a devpi mirror. Unfortunately our build system requires us to install from an sdist rather than a wheel.

Ideally this would be backported to the v2.2.x branch. Should I submit a separate pull request for that?

PR Checklist

• [N/A] Has Pytest style unit tests
• [/] Code is Flake 8 compliant
• [N/A] New features are documented, with examples if plot related
• [N/A] Documentation is sphinx and numpydoc compliant
• [N/A] Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• [N/A] Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[tacaswell]

Thanks @AWhetter !

[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 v2.2.x
$ git pull

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

$ git cherry-pick -m1 b49ca9dd8a0a6cf1bda872ffe0c73025af1f9bf2

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

$ git commit -am "Backport PR #14915: Don't redownload jquery when installing from an sdist"

4. Push to a named branch :

git push YOURFORK v2.2.x:auto-backport-of-pr-14915-on-v2.2.x

5. Create a PR against branch v2.2.x, I would have named this PR:

"Backport PR #14915 on branch v2.2.x"

And apply the correct labels and milestones.

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

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

[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 v3.1.x
$ git pull

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

$ git cherry-pick -m1 b49ca9dd8a0a6cf1bda872ffe0c73025af1f9bf2

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

$ git commit -am "Backport PR #14915: Don't redownload jquery when installing from an sdist"

4. Push to a named branch :

git push YOURFORK v3.1.x:auto-backport-of-pr-14915-on-v3.1.x

5. Create a PR against branch v3.1.x, I would have named this PR:

"Backport PR #14915 on branch v3.1.x"

And apply the correct labels and milestones.

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

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

[tacaswell]

The conflicts on both branches were with import order so fortunately super easy to resolve!