Re: GHC packages' inputs leak in guix shell

[Saku Laesvuori]

> I can’t check right now, but I’m guessing a plain `cabal install`
> would also add base64-bytestring to GHC’s visible packages?

I tested with cabal-install and it somehow manages to hide the haskell
packages that are installed as dependencies.

```
$ guix shell -CN cabal-install coreutils zlib -D ghc-old-time
## Building old-time failed so I added it's inputs.
## Zlib is also required for building the dependency chain.
$ cabal update
$ env -u GHC_PACKAGE_PATH cabal install --lib esqueleto
$ ghci
ﬦ import Database.Esqueleto.Experimental
-- This works, so the package from cabal-install is visible
ﬦ import Data.ByteString.Base64.URL.Lazy

<no location info>: error:
    Could not load module `Data.ByteString.Base64.URL.Lazy'
    It is a member of the hidden package `base64-bytestring-1.2.1.0'.
    You can run `:set -package base64-bytestring' to expose it.
    (Note: this unloads all the modules in the current scope.)
```

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