Improve support for package-with-macro-application

[srawlins]

The previous fix is no good; it was concerned with augmentations, assuming they cannot be read from disk. This is not the case. Only a macro-generated augmentation cannot be read from disk.

Instead we change tactics and say we can include source code from a macro (maybe we shouldn't), and just ask analyzer for all source files, from the AnalysisContext, instead of going straight to disk.

Other changes to facilitate this:

• Do not dispose the AnalysisContextCollection right away; we should wait until all docs are generated.
• Pass the AnalysisContext to ModelNode, and no longer pass the ResourceProvider.
• Remove the file cache thing in model_utils; instead ModelNode.sourceCode only relies on the AnalysisContext. Theoretically there is already caching over that at analyzer? But I benchmarked with Flutter and saw no time-to-document or max-RSS changes.
• Do not ask the AnalysisContextCollection for the AnalysisContext for every file! Cache it. To facilitate that, change PubPackageBuilder() into a factory.
• Sort the fields instantiated by the PackageGraph.uninitialized constructor above it.
• In addFilesReferencedBy, also look at augmentation imports.

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