Add cabal get --only-package-description

[hvr]

With this option, 'cabal get' writes to the destination directory only the
package description already available locally in one of the repository
indices.

The basename of the file name written to inside the target directory is
the package-id rather than only the package name.

This is mostly based on #1977

Co-authored-by: Miëtek Bak mietek@bak.io

---

Please include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.

Please also shortly describe how you tested your change. Bonus points for added tests!

[23Skidoo]

Looks all right on a first glance, but needs docs; this is the relevant manual section: https://cabal.readthedocs.io/en/latest/developing-packages.html#downloading-a-package-s-source

[hvr]

@23Skidoo just asking for the changelog-entry, is this 2.2 material or post-2.2 ?

[23Skidoo]

I think it's low risk, we can merge it into 2.2.

[hvr]

@23Skidoo fwiw; I still need to get --pristine working -- --index-state already works out of the box, but --pristine doesn't yet (this is what blocked #1977 from getting merged originally). Depending on how I go about this, this may actually be more invasive than would be justifiable for 2.2 :-)

[jneira]

if this is still sensible I think pristine should be already working (I hope so, don't checked it)

[jneira]

This needs a changelog entry and updating docs but the code looks good

[jneira]

oops

[jneira]

Closing in favour of #8263, thanks all!