Remove Indexable as an unnecessary mixin.

[srawlins]

This mixin only served as an interface providing href and kind, and providing an impl for overriddenDepth. This amounted to a lot of complexity. Some APIs were thus written in terms of Indexable. And any class that mixed in Indexable gets an overriddenDepth property which is inapplicable, more often than not.

As Documentable has href and kind, the easiest migration is to switch most Indexable-concerned APIs to use Documentable. We move overriddenDepth to only be defined on Inheritable.
below.

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[srawlins]

Ready for review, @dart-lang/analyzer-team but zero rush 😀