fix: fail if superfluous packages are excluded

[SMillerDev]

Fixes issue where:

• you add a package and depend on the six formula so you add it to the exclude list
• upstream removes the dependency on six in a new release
• everything keeps depending on six since currently we have no way to know if the excludes list has superfluous entries.

[MikeMcQuaid]

Can you provide a bit more context for what's happening here? What's a superfluous package and why is excluding one fatal?

[SMillerDev]

So the thing I'm trying to fix is:

• you add a package and depend on the six formula so you add it to the exclude list
• upstream removes the dependency on six in a new release
• everything keeps depending on six since currently we have no way to know if the excludes list has superfluous entries.

[MikeMcQuaid]

Thanks for elaborating! Have put your explanation into the PR body so it'll be in a merge commit. One suggestion and then, when CI fixed, can approve and merge 👍🏻