`cabal unpack hackport-0.5.3` fails because of syminks in tarbal

The package tarball

https://hackage.haskell.org/package/hackport-0.5.3/hackport-0.5.3.tar.gz

fails to unpack with `cabal unpack` with the error below (consequently, also `cabal install hackport-0.5.3` will likely fail!):

```
cabal unpack hackport-0.5.3
Unpacking to hackport-0.5.3/
hackport-0.5.3/dist/build/hackport/hackport: copyFile:atomicCopyFileContents:withReplacementFile:copyFileToHandle:openBinaryFile: does not exist (No such file or directory)
```

This .tar.gz seems to have been created manually, and when inspecting its contents you find a symlink

```
lrwxrwxrwx slyfox/users 0 2017-05-24 22:54 hackport-0.5.3/hackport -> dist/build/hackport/hackport
```

which I suspect may be the reason for this. But this needs confirmation. Be that as it may, `cabal` should handle this more gracefully imho.