Getting package contents fails for some files

I ran into a problem trying to browse the source of a package that has an unusual tarball. Trying to visit returns the following HTTP 500 response:

> Happstack 7.5.4
>
> Something went wrong here
> Internal server error
> Everything has stopped
>
> The error was "user error (failed to read entry from tar file)"

I think this happens because the tarball has a hard link to a file that already exists in the archive. I wrote the following script to get the tarball and dump out a summary of its contents:

``` hs
#! /usr/bin/env stack
-- stack --resolver lts-14.21 script
import qualified Codec.Archive.Tar as Tar
import qualified Codec.Archive.Tar.Entry as Tar
import qualified Codec.Compression.GZip as Gzip
import Data.Function ((&))
import qualified Network.HTTP.Client as Client
import qualified Network.HTTP.Client.TLS as Client

main = do
let url = "https://hackage.haskell.org/package/edit-lenses-demo-0.1/edit-lenses-demo-0.1.tar.gz"
request <- Client.parseUrlThrow url
manager <- Client.newTlsManager
response <- Client.httpLbs request manager
response
& Client.responseBody
& Gzip.decompress
& Tar.read
& Tar.foldEntries (:) [] (error . show)
& zip [ 0 .. ]
& mapM_ (\ (index, entry) -> putStrLn
$ show index
++ ": "
++ Tar.entryPath entry
++ " => "
++ case Tar.entryContent entry of
Tar.BlockDevice _ _ -> "BlockDevice"
Tar.CharacterDevice _ _ -> "CharacterDevice"
Tar.Directory -> "Directory"
Tar.HardLink target -> "HardLink " ++ Tar.fromLinkTarget target
Tar.NamedPipe -> "NamedPipe"
Tar.NormalFile _ _ -> "NormalFile"
Tar.OtherEntryType code _ _ -> "OtherEntryType " ++ [code]
Tar.SymbolicLink target -> "SymbolicLink " ++ Tar.fromLinkTarget target)
```

Running that and grabbing the relevant lines from the output produces:

```
26: edit-lenses-demo-0.1\Data\Module\String.hs => NormalFile
87: edit-lenses-demo-0.1\Data\Module\String.hs => HardLink edit-lenses-demo-0.1\Data\Module\String.hs
99: edit-lenses-demo-0.1\Data\Module\String.hs => HardLink edit-lenses-demo-0.1\Data\Module\String.hs
```

This is similar to #851 in that both involve weird tarballs.

Start by reproducing the package URL and the supplied tarball inspection script, then trace the server's package-content handling for hard-link entries. Compare the behavior with issue #851. Done means the affected source file can be opened without an HTTP 500 error.

Written by the indexing model from the issue text.