Replace cabal project parsing with Parsec

[jgotoh]

Implements #6101

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• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
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This is a really early version to gather some feedback and to check whether I am on the right track.
I created the rough module structure and a draft of the parser with still lots of holes.
My next steps will be the definition of the FieldGrammar and Lens for ProjectConfigs, afterwards I want to add lots of tests for it.
There are some open questions in the source code that I am really unsure of, I will create some comments after I have created the PR.

[jgotoh]

@ulysses4ever as announced, here is an early PR of the changes. Still lots of work, but I have a rough outline of the ticket. I would be glad if you would take a look!

[ulysses4ever]

Thanks @jgotoh ! Will take a look eventually.

@gbaz you know this code better than anyone currently active, i think. Would you be able to provide guidance?

[ulysses4ever]

@grayjay I know you’re more of a solver person, but if you happen to have some time for advising and feel comfortable in this part of the code, your contribution would be priceless.

[ulysses4ever]

@jgotoh there's a bi-weekly cabal devs meeting, where, I am sure, people would be delighted to hear your experience so far. The closest one is this Thursday (Apr 20th), 1 PM Eastern Time (US). The link to a Jitsi video call is posted before the meeting on #hackage at libera.chat (can be browsed using Element/Matrix or an IRC client). Are you interested?

[jgotoh]

@ulysses4ever Thank you very much for you invitation! I've already wondered about the cabal devs main communication channel. I am definitely interested and will be glad to attend :)

[ulysses4ever]

@jgotoh cool! let me know if you want me to mail you the jitsi link beforehand.

[grayjay]

@grayjay I know you’re more of a solver person, but if you happen to have some time for advising and feel comfortable in this part of the code, your contribution would be priceless.

I'm not familiar with this part of the code, but I tried to answer a couple questions based on my understanding of the parser behavior.

[jgotoh]

@gbaz I migrated this from Legacy.hs:281 to the new ParseResult. I think you implemented the old version in Legacy.hs? Could you help reviewing this new version? As far as I see it works, but it would be awesome if you would take a look.