set plugin development find-links only on the needed resolve

[cburroughs]

As described at
#21223 (comment), if you set find-links (and that includes everyone with in-repo plugins by way of pants.backend.plugin_development) on one resolve, we were erroneously setting it for all resolves during lockfile generation. This materially impacts resolution time.

[cburroughs]

(Back to draft, found a complication.)

[cburroughs]

Thanks for the contribution. We've just branched for 2.31.x, so merging this pull request now will come out in 2.32.x, please move the release notes updates to docs/notes/2.32.x.md if that's appropriate.

[cburroughs]

(Back to draft, found a complication.)

The code was correct, but my description/changelog was wrong and mislead me when I looked at it later. This change fixes the narrow issue of pants.backend.plugin_development injecting into all resolves, but [python-repos].find_links itself is still global and unchanged.

[tobni]

I hav everified this as working (the index is appropriately scoped), but without any performance improvement. Maybe because my company internal mirror has the same issue as the pants find_links.

[jsirois]

Maybe because my company internal mirror has the same issue as the pants find_links.

@tobni yes. Exactly all indexes in use in a resolve must support PEP-658 if you want to get faster resolves (due to lack of wheel downloads to learn metadata).