Require a revision 0 cabal file when force-version is used

[neilmayhew]

Instead of rewriting the cabal file, we use an archived version that's stored as revision 0. This avoids changing existing cabal files when Cabal's pretty-printing changes.

[neilmayhew]

This works in tandem with IntersectMBO/cardano-haskell-packages#1256 which modifies the add-from-github.sh script to create revisions/0.cabal when forcing a version.

Although the script uses a text-substitution approach, rather than the AST-based rewriting that was previously done by foliage, I think that's relatively robust because the text substitution is being done when the script is run interactively by a human, and it displays a diff of the cabal file afterwards so the user can see exactly what was changed (and it should be only the version number). In other words, there's some human oversight. After that, the file is committed to git and will never change again.

The only change to foliage here is to require a 0.cabal file from the git repo, instead of rewriting the file from the tarball during the build. I actually feel safer with this than with a reformatting of the file that makes it very difficult for a human to check what has changed.

[andreabedini]

This is good. I wonder if we simplify the process.

• deprecate force-version
• pkg-name/pkg-version/revisions/0.cabal always replaces the cabal file when it exists.

What is annoying is that we have the package name and package version in the path already. As it stands foliage simply checks that the two sources of package version (the path and the metadata) matches but TBH I find this unsatisfactory. Nevertheless, this change does not improve or regress w.r.t to this duplication, so that is not a problem.

[andreabedini]

This looks good as it stands.

[angerman]

Alright let's do this then.

[neilmayhew]

Thanks for the review, Andrea.

• deprecate force-version
• pkg-name/pkg-version/revisions/0.cabal always replaces the cabal file when it exists.

I agree that this would be a good improvement. Then, force-version simply becomes an artifact of the add-from-github.sh script.

I'll create a PR for this.