Move the non-Candidate `packageRender` from `RecentPackages` to `PackageContents`.

This eliminates unnecessarily threading `findToplevelFile` through `PackageContents`. The other solution would be to have `RecentPackages` depend on `TarIndexCache` directly.
Add (untested!) support for loading readme files from tarball.

The pattern files are tested against is in `Packages.Readme`. Both candidates and normal packages have the contents of both their Changelog and Readme incorporated into `PackageRender`.