Improve error message for empty --allow-newer= (fix #7740)

[ulysses4ever]

This is a WIP, it's not beautiful: (see the first comment below)

❯ $CABAL build --allow-newer=                                                                                              1 (0.018s) < 04:31:50
Error: cabal: invalid argument to option `--allow-newer': "<parsecToReadE>"
(line 1, column 1):
unexpected empty argument list for --allow-newer=. Use --allow-newer without
the equals sign to permit all packages to use newer versions.
expecting "*"

Should "parsecToReadE" be removed? It can be but this may make it harder to debug anything that goes via parsecToReadE.

expecting "*" is annoying but I'm not sure how to remove it either.

---

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[ulysses4ever]

Okay, I think it's ready for review now. I found a way to extend the current approach for converting ParsecParser to ReadE that shouldn't break anyone's workflow, and at the same clean up the parsing error message:

❯ $CABAL build --allow-newer=                                                                                                    1 (0.029s) < 04:55:15
Error: cabal: invalid argument to option `--allow-newer': empty argument list
is not allowed. Note: use --allow-newer without the equals sign to permit all
packages to use newer versions.

I didn't notice that error message were tested anyhow. Have I missed it and should add a test? At the moment I'm not sure where to add it.

[ulysses4ever]

Would appreciate if @phadej could take a look, as the parts of code I touched are mostly due to him.

[Mikolaj]

A tiny changelog file would be needed nevertheless. LGTM otherwise. Congrats.

[ulysses4ever]

@Mikolaj done! Thanks for the review!

[Mikolaj]

CI got canceled. Let me try to restart.

[Mikolaj]

@Mergifyio rebase

[mergify]

rebase

✅ Branch has been successfully rebased

[Mikolaj]

No time to lose --- let's merge it (via merge_me label) before somebody says it doesn't make sense.

[Kleidukos]

Jetzt geht's los!