Convert many fields on Container and TopLevelContainer to getters

[srawlins]

These were late, final, and public, my least favorite thing. The idea, historically, behind late final fields in dartdoc is caching. If a value is expensive to calculate, like a big list of things, and we have to access that value a few times, best to calculate it once.

But some late final fields are only accessed while rendering HTML, accessed from the rendered templates. And typically only accessed once. The '*Sorted' fields definitely fall into this bucket. So I made them all getters, and then benchmarked with the googleapis package, and found no significant change in time-to-document, or max RSS.

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