clean up: consolidate documentLoader and didResolvers

[aljones15]

1. the documentLoader should be an async function that accepts a url and returns a document
2. the existing documentLoader in the tests dir has a parameter {url} that should just be url
3. the existing documentLoader in the tests dir does not use jsonld-document-loader
4. the existing documentLoader uses didResolver.js which can be rolled into jsonld-document-loader
5. increasingly this project has multiple documentLoaders these can be consolidated into a single function that creates a documentLoader, that function should come from data-integrity-test-suite-assertion or vc-test-suite-implementations at some point so other projects can create a documentLoader with support for a particular did:key type.
6. the documentLoader in tests should be removed or moved somewhere else