Fix search which is not using canonical libraries as enclosing elements

[srawlins]

Search is broken at api.dart.dev/dev, e.g. https://api.dart.dev/dev/3.6.0-228.0.dev/dart-collection/Queue-class.html

This fixes that. I move the assertions from the code that reads the index.json file to the code that writes the index.json file. I also use a library member's canonical library as it's enclosing element in the search results.

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[srawlins]

Ready for review, @dart-lang/analyzer-team

[bwilkerson]

I have no idea what I'm looking at, but I also don't see anything obviously questionable.

[srawlins]

Nothing obviously questionable is good enough for me :D