Limit file count and total length in pkg/pub_dartdoc.

[isoos]

• dartdoc runs too long - possible patterns dartdoc#2615
• written files that are inside the output directory are kept in memory with a strict limit (10k files, 10MB total size)
• this is a better approach than overriding only the DartdocFileWriter, as the later misses the static files that are copied directly
• updated pub-dartdoc-data to use the same resource provider pattern
• files are written to the disk at the end of the process

[jonasfj]

Why not just create a proper overlay, instead of relying on output folder detection?

When writing we always write to _memoryResourceProvider, when reading we read from _memoryResourceProvider and if not present there, then we check: _defaultProvider.

I guess the complicated part is when listing folders, we'll need to wrap and merge them... But it would be less magic..

We can also leave that as a TODO... (I'm guessing we can almost just steal the overlay implementation from analyzer and binary file support to it, why not?

[isoos]

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