[UX] Project Installer: On each cron run, pull and cache locally a project list from b.org

...so that we can do a quick first search.

This is part of #1288.

The point is to have the project search on /admin/modules/install work like the instant search that's available on the List modules page (/admin/modules). Perhaps do an instant search from the local cache while a fresh list of more projects is fetched in the background?