Add providers to organisation repo

[giggsey]

The PR's related to this have sped up our installs (on busy projects with a mix of organisation / public packages) from ~600s to ~60s. But our satis setup still does it in 30s.

Additionally, composer v2 supports an 'available-packages' array, which would help when using an org repo followed by the packagist mirror repo.

Originally posted by @giggsey in #261 (comment)

[giggsey]

Actually, I think the providers-includes can be skipped.

composer v2 doesn't use provider-includes, and it's due for release this month. I think there are separate cache improvements that can be made to the Repo pages. That mixed with available-packages should provide a good speed improvement for composer v2.