Use Distribution.Fields.Pretty in `cabal init`.

[phadej]

This will make output of cabal init and cabal format closer to each other.

[phadej]

One way to solve this is to first generate GenericPackageDescription of a new package,
process it down to [PrettyField ()] and attach comments to relevant fields in ad-hoc post-processing step.

This won't if cabal init ought to generate common stanzas, or some other structure not visible in GPD.

This way cabal format on non-commented cabal init file would be no-op.

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An alternative, to achieve cabal format and cabal init compatibility, is to change cabal format to work like cabal-fmt, i.e. only on Field/PrettyField; not going down to GPD at all. If someone thinks that's a way to go, please open an issue and argue why "through GPD" is not required.