Setup.hs depends on parsec, but doesn't declare it

The `Cabal.cabal` file has to declare `parsec` as a dependency of the setup script. This problem exists in current versions of `master` as well as in the `2.2.0.1` release.

~~~
$ ghc --make Setup.hs
[ 1 of 217] Compiling Distribution.Compat.Binary ( Distribution/Compat/Binary.hs, Distribution/Compat/Binary.o )
[ 2 of 217] Compiling Distribution.Compat.Directory ( Distribution/Compat/Directory.hs, Distribution/Compat/Directory.o )
[ 3 of 217] Compiling Distribution.Compat.Exception ( Distribution/Compat/Exception.hs, Distribution/Compat/Exception.o )
[ 4 of 217] Compiling Distribution.Compat.Internal.TempFile ( Distribution/Compat/Internal/TempFile.hs, Distribution/Compat/Internal/TempFile.o )
[ 5 of 217] Compiling Distribution.Compat.Map.Lazy ( Distribution/Compat/Map/Lazy.hs, Distribution/Compat/Map/Lazy.o )
[ 6 of 217] Compiling Distribution.Compat.Map.Strict ( Distribution/Compat/Map/Strict.hs, Distribution/Compat/Map/Strict.o )
[ 7 of 217] Compiling Distribution.Compat.MonadFail ( Distribution/Compat/MonadFail.hs, Distribution/Compat/MonadFail.o )
[ 8 of 217] Compiling Distribution.Compat.Newtype ( Distribution/Compat/Newtype.hs, Distribution/Compat/Newtype.o )
[ 9 of 217] Compiling Distribution.Compat.Semigroup ( Distribution/Compat/Semigroup.hs, Distribution/Compat/Semigroup.o )
[ 10 of 217] Compiling Distribution.Compat.Stack ( Distribution/Compat/Stack.hs, Distribution/Compat/Stack.o )
[ 11 of 217] Compiling Distribution.Compat.Prelude ( Distribution/Compat/Prelude.hs, Distribution/Compat/Prelude.o )
[ 12 of 217] Compiling Distribution.Compat.ReadP ( Distribution/Compat/ReadP.hs, Distribution/Compat/ReadP.o )
[ 13 of 217] Compiling Distribution.Compat.Parsing ( Distribution/Compat/Parsing.hs, Distribution/Compat/Parsing.o )

Distribution/Compat/Parsing.hs:61:1: error:
Could not find module ‘Text.Parsec’
Use -v to see a list of the files searched for.
|
61 | import qualified Text.Parsec as Parsec
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
~~~