Remove 'ExecutableScopeUnknown'.

[phadej]

Fixes #5083
Related to #5003

Note: buildable: field has All semantics. scope: has
AnyPrivate semantics. Both might be surprising, but are "logical".

https://hackage.haskell.org/package/Cabal-2.0.1.1/docs/src/Distribution.PackageDescription.Parse.html#line-248

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[23Skidoo]

We didn't allow users to write scope: unknown, so this is backwards compatible, right? Otherwise LGTM.

[phadej]

ReadP parser is

instance Text ExecutableScope where
      parse = Parse.choice
         [ Parse.string "public"  >> return ExecutablePublic
         , Parse.string "private" >> return ExecutablePrivate
         ]

so cabal-2.0 didn't accept anything else than public and private. I think I made Parsec more lenient to allow roundtrip, so it (= removal or pretty-printer fix) was forgotten to do item.

[23Skidoo]

OK, let's merge.

[phadej]

Failure is legit, added test in master "conflicts". EDIT: amended

[23Skidoo]

Merged, thanks!