Refactor "could apply" logic for extensions

[srawlins]

This logic was implemented in a few places between Extension and ElementType (and subtypes). We don't need any logic to live in ElementTypes though, since any calculations that need to be made regarding DartTypes or InterfaceTypes can be made in Extension (alwaysApplies and couldApplyTo). This fixes #3846 and dramatically simplifies the implementation. Also:

• Rename Extension.extendedType to Extension.extendedElement.
• Change PackageGraph._extensions to a Set, to remove duplication in multiple canonical libraries.
• Introduce a classes test.
• Delete some redundant end2end testing code.

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[srawlins]

CC @dart-lang/analyzer-team