support cabal new-build

[gelisam]

hawk uses its executable's location to guess where the corresponding package database is. This works with cabal sandboxes:

$ cabal sandbox init
$ cabal install
[...]
$ cabal exec hawk '2+2'
4
$ .cabal-sandbox/bin/hawk '2+2'
4

and with stack:

$ stack build
$ stack exec hawk '2+2'
4
$ .stack-work/dist/x86_64-osx/Cabal-2.0.1.0/build/hawk/hawk '2+2'

but not with cabal new-build:

$ cabal new-build
$ ./dist-newstyle/build/x86_64-osx/ghc-8.2.2/haskell-awk-1.1.1/build/hawk/hawk '2+2'
hawk: No package-db found. Did you install Hawk in an unusual way?
CallStack (from HasCallStack):
  error, called at src/System/Console/Hawk/Sandbox.hs:81:14 in main:System.Console.Hawk.Sandbox