ComposerRepository::findPackage[s]

I'm not sure why `ComposerRepository::findPackage[s]` is not implemented. The `ArrayRepository` implementation will end in a `Composer repositories that have providers can not load the complete list of packages, use getProviderNames instead` exception, if you try to use `ComposerRepository::findPackage[s]`.

I implemented it based on the `ComposerRepository::whatProvides` method, I'm not sure if this is the right way, but it works.