Deprecate public exports in lib/dartdoc.dart.

[kallentu]

Public exports deprecated for 8.0.1 bump eventually and will be fully removed in 9.0.0.

• Updated the tests to avoid using the import.

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[srawlins]

I guess we should deprecate this file too.

[kallentu]

All the classes in the file?
We should eventually just move them to src/dartdoc_options.dart or something

[kallentu]

I'll merge this one first and I can get another commit going for options.dart :D