Frozen dependencies are printed on their own lines

A new pretty-print has been introduced `D.ParseUtils.commaNewLineListField`.  It is similar to `D.ParseUtils.commaListField` but has each item on its own line.

The constraints section of the package environment is pretty printed with `commaNewLineListField`, but no other fields have been changed.

The constraints field was already sorted albeit case sensitvely. I have left that untouched. When I freeze the dependencies for cabal-install the cabal.config file is:

```
constraints: Cabal ==1.19.2,
             HTTP ==4000.2.11,
             array ==0.4.0.1,
             base ==4.6.0.1,
             bytestring ==0.10.0.2,
             cabal-install ==1.19.2,
             containers ==0.5.0.0,
             deepseq ==1.3.0.1,
             directory ==1.2.0.1,
             filepath ==1.3.0.1,
             ghc-prim ==0.3.0.0,
             integer-gmp ==0.5.0.0,
             mtl ==2.1.2,
             network ==2.4.2.2,
             old-locale ==1.0.0.5,
             old-time ==1.1.0.1,
             parsec ==3.1.5,
             pretty ==1.1.1.0,
             process ==1.1.0.2,
             random ==1.0.1.1,
             rts ==1.0,
             stm ==2.4.2,
             text ==1.1.0.1,
             time ==1.4.0.1,
             transformers ==0.3.0.0,
             unix ==2.6.0.1,
             zlib ==0.5.4.1

install-dirs 
```