Remove FetchResources trait (breaking)

[foresterre]

• Fetching resources is often dependent on what is being fetched
• Trying to be generic over this is better left of to those who depend on this crate
• Especially since they often will only use a subset, or even only one client
• In addition, letting each client sort this out for themselves allows us to also

This is also the first of many PR's of a larger refactor with the following goals

• Merging sources: sometimes we want to combine multiple sources to come to a better release index. For example, some sources are updated quicker after a release is published than others, some are easier to fetch, but are also incomplete with respect to data they contain. The merged index should have a more complete view than the individual parts.
  • Consider different merging strategies; we don't always have data with timestamps, and not always is later also more correct.
• A built in source of truth index for releases in the past, baked statically into a crate (this is mostly useful if you can merge it with newer data)
• More metadata: currently we only cared about versions, and let dependencies gather additional metadata themselves, sometimes in an unpure way of getting it from the cache collected by this crate
• More than just stable: so far we only implemented fetching stable releases, but we care also about beta and nightly releases. We will skip the early alpha and other pre-1.0 releases.