Remove unnecessary ExtensionTarget mixin

[srawlins]

This mixin just adds complexity; it is only mixed into one class, and then stubs out some abstract members; it's a lot of ceremony.

Easy enough to fold it into InheritingContainer, with its two concrete members.

Also:

• Make Extension.declaredMethods final.
• Move the InheritingContainer constructor up.
• Reorganize code in InheritingContainer.publicInterfaces to be more readable; move the assert closer to the condition that triggers it.

Work towards #2021

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[kallentu]

Love removing mixins. Thanks!

[parlough]

Love removing mixins. Thanks!

As long as they aren't icecream mix-ins!