The API was not ideal, as it meant the caller has
to be a bit repetitive in what it was doing:
- defaultPlacementLocation should be used when possible;
- A library user would most likely want FetchOptions.none;
- The most simple use case would be 'dub.fetch(pkgName)',
  meaning get me the latest version of pkgName, as is used
  by the tools fetching code;
All of this is now supported, in addition to a transition
to using the PackageName API.