cabal-version: >=1.7.0

With cabal versions prior to 1.7.1 the build-depends of any executable stanzas
are included in the build depends which are registered with ghc-pkg. Cabal's
resolver therefore has a complete set of build-depends when it selects the
installed version of foo. `./test.sh` does not exhibit the bug described in
https://github.com/haskell/cabal/issues/1896.
cabal-version: >=1.7.1

With cabal version's greater than or equal to 1.7.1 the build-depends of any
executable stanzas are not included in the build-depends registered with
ghc-pkg. This causes `./test.sh` to exhibit the bug in
https://github.com/haskell/cabal/issues/1896.
Remove library stanza

If the library stanza is removed the package is not registered with ghc-pkg.
This prevents the cabal's resolver from using the incomplete set of
build-depends that ghc-pkg knows about. `./test.sh` no longer exhibits the bug
described in https://github.com/haskell/cabal/issues/1896.