Do not send composer v1 provider-list in provider field anymore

[pascalheidmann]

This pull request makes the following changes:

• removes package list from packages.json endpoint as composerV2 does not use it

[stklcode]

Did v1 actually use it? Wondering if providers was ever correct in this place. Docs name provider-includes which points to JSON includes that contain the providers list, but no list directly in the repo spec.

We still provide providers-url for v1, so honestly I can’t remember if it ever made any difference or if providers was always ignored.

Nevertheless, if we do not generate the providers from packages, we also don’t have to parse the package list on proxy repos, which is optional and not available on all repos (see #150).

While refactoring this I also eliminated providers on hosted repos and added support for available-package(-pattern)s instead (see #165)