[pigeon] fixed cases where types can be missing from codecs

[gaaclarke]

Previously we fixed that classes are generated when referenced only from type arguments, we were still missing them from the codec though. This fixes that.

fixes internal question 4652416327166722048

Pre-launch Checklist

• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
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[stuartmorgan]

LGTM

[ditman]

I'll land this after #469 lands to see if the repo goes back to green!

[ditman]

Landing to test the auto-publish script.

[ditman]

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Tagging release pigeon-v1.0.4...
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Published pigeon successfully!

[gaaclarke]

Thanks @ditman, I actually couldn't sleep so I decided to try to get this landed since it was bugging me. I'm happy to see it already done. Guess I'll go back to staring at the wall haha.

[ditman]

Apologies @gaaclarke for depriving you of this midnight pastime! (Landing this was a very convenient way of testing the CI on packages/plugins, that had been on fire for the whole day!)