Upgrade to hpack 0.20.0 (fixes #3383)

[snoyberg]

Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

Please include the following checklist in your PR:

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

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

[mgsloan]

I think "please delete it" would be clearer, right now it could read as "delete + modify package.yaml". Perhaps it's not so ambiguous, but still best to be clear as possible

[sol]

Or something along the lines of

"WARNING: $cabalFile was modified manually.  Ignoring package.yaml in favor of $cabaFile.  If you want to use package.yaml instead of $cabalFile then please delete $cabalFile."

?

[mgsloan]

Hmm, perhaps! Or it could just pre-emptively delete the cabal file! 👿 Hah probably people wouldn't appreciate that.

Perhaps it should just error in this circumstance. Hmm hmm, nothing seems to be quite perfect.

[sol]

One more note: The reason why I added --force to hpack and why --force is preferable to deleting the cabal file is, that hpack does some effort to minimize diffs if a cabal file is already present. So hpack --force and rm *.cabal && hpack may lead to differently formatted cabal files.

[snoyberg]

I'll go with @sol's verbiage, I think the hpack author is likely the best to figure these things out anyway :)

[mgsloan]

LGTM just one comment