Address comments on #2949.

This changes the definition of `Index` to

``` haskell
data Index =
    -- | The main index for the specified repository
    RepoIndex Repo

    -- | A sandbox-local repository
    -- Argument is the location of the index file
  | SandboxIndex FilePath
```

with

```
cacheFile (SandboxIndex index) = index `replaceExtension` "cache"
```

This also renames `repoRemote'` to `maybeRepoRemote`.

I believe this addresses all comments.