Make spago verify-set fail if there are duplicate module names.

[Dretch]

This is needed for https://github.com/purescript/package-sets/issues/490

Note that spago verify <package> does not do the check, only spago verify-set does. This is because the check requires downloading and compiling all packages, which would make spago verify <package> almost as slow as spago verify-set.

Checklist:

• Added the change to the "Unreleased" section of the changelog
• Added some example of the new feature to the README
• Added a test for the contribution (if applicable)

[f-f]

@Dretch looks great to me! 👏

@justinwoo could you take a look?

[justinwoo]

looks like it does what is desired. i don't think this is necessarily what someone who manages their own package set will want to do, but in those cases, someone might use dhall-to-json <<< packages.dhall | jq 'keys[]' -r | xargs -I% spago verify % or something as needed.

[f-f]

@justinwoo great point. We might want to add a --skip-duplicated-module-check flag then? (it's not an actual suggestion for the flag name, but just to give the idea)

[f-f]

💯