Resolve #6359

`preferred-versions` are left out for now.
It shouldn't be difficult to add, but needs work nevertheless.

We also allow relative paths, which kind of work,
if you are careful.

In addtition
- change the index cache to use `Distribution.Utils.Structured`,
  making Binary instances generically derived.
- separate Distribution.Client.HashValue into own module.
  This allows to use HashValue for hashing the part of localRepoPath
  (breaks module dependency cycle).

Almost as a feature generated 01-index.cache is never updated.
If you change the contents of the directory, you have to purge
01-index.cache file yourself.