Refactor some code in PackageGraph

[srawlins]

Two things:

• The findOrCreateLibraryFor method is essentially a long way of writing "putIfAbsent". Remove and replace calls with allLibraries.putIfAbsent.
• Make the specialClasses field final.
• Refactor _precacheLocalDocs to not use sync*; notoriously slow. Simple migration. The result is always iterated completely.

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