cabal sdist: Rename Cabal file to match the package name

[mmhat]

This PR modifies how source distribution are packaged by cabal-install:exe:cabal:

Before, if the name of the Cabal file did not match the actual package name, then cabal sdist would include the Cabal file as-is in the source distribution, causing a subsequent cabal check to issue a warning and the failure of a Hackage upload.

This commit changes that behaviour: The name of the Cabal file in the source distribution always matches the package name, regardless what it was named in the filesystem.

EDIT:
Related issue: #6299
Fixes #7322

---

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
  • Is the change significant? If so, remember to add significance: significant in the changelog file.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)

[ulysses4ever]

It feels to me like it can break someone's workflows, i.e. people may assume that the name of the cabal file does not magically change --- a very reasonable expectation! Maybe the new behaviour could be enabled by a flag?.. In any case, there should be an issue with the discussion of the design before a PR, see https://github.com/haskell/cabal/blob/master/CONTRIBUTING.md#pull-requests--issues

[geekosaur]

If Hackage is currently rejecting such packages, it's probably not breaking many workflows.

[mmhat]

First, thanks for your feedback @ulysses4ever @geekosaur !

It feels to me like it can break someone's workflows, i.e. people may assume that the name of the cabal file does not magically change --- a very reasonable expectation! Maybe the new behaviour could be enabled by a flag?

I see your concerns, and I am not sure about that myself; That's why this is a draft PR.
However, I agree with @geekosaur that there are probably not that many workflows (if any) this would break, but not renaming the Cabal file caused already issues; See #7322.

In any case, there should be an issue with the discussion of the design before a PR, see https://github.com/haskell/cabal/blob/master/CONTRIBUTING.md#pull-requests--issues

I am aware of the guide lines, but I also forgot to link two related issues #6299 and the aforementioned #7322 in the description of the PR; Sorry for that.
Do you think one of these issues is sufficient or should I open a separate one?

BTW: The PR template proposed to me referred to contribution guidelines found in cabalism/cabal -- Is this intended?

[mpickering]

You could probably pass in the location, since you must have already read the cabal file, but I do see that listPackageSources does also call tryFindPackageDesc.