Run and process dartdoc output to create pub-data.json and the coverage report.

[isoos]

• part of [meta] pana standalone should be the same as pub.dev output #1229, Pana shows reduced score while running locally #837 and Indicate which elements are not documented in pana report #979
• copied most files in lib/src/dartdoc from pub-dev (except lib/src/dartdoc/dartdoc.dart)
• enabled to run the analysis by default in bin/pana.dart, using the latest dartdoc through (a) custom PUB_CACHE + pub global active dartdoc
• enabled to run on a few selected packages in end2end tests (also updated end2end reference timestamp and output)

[isoos]

PTAL. I've removed the retry loop, deprecated its option and refactored the package_context code as we have discussed. I've opted for another small breaking change with DartdocResult, as we haven't used that class for anything recently.

[sigurdm]

LGTM