Allow fine-grained warning control with flags and dartdoc_options.yaml files

[jcollins-g]

Fixes #1343, the last long-running dartdoc P1 bug. Fixes #1412 by squelching warnings irrelevant to the package you're documenting. Fixes #1480 by obsoleting it.

The last part of #1343 is providing a way to control warning displays similar to the analyzer and linter, including allowing for a hard-failure option. This PR does that.

Changes:

• By default, packages not being documented no longer display their warnings or errors. This can be overridden with --allow-non-local-warnings. (further tighten warnings to eliminate false positives #1412)
• New command line flags --allow-warnings-in-packages, --allow-errors-in-packages, --ignore-warnings-in-packages, and --ignore-errors-in-packages will allow large, multi-package documentation sets to narrow their field of view. Particularly important for Flutter docs.
• New command line flags --errors, --warnings, and --ignore allow direct control of which warnings are handled in what manner.
• dartdoc_options.yaml allows errors, warnings, and ignore to be applied to subdirectory trees, so packages can specify what warnings they care about. Command-line arguments supercede these.
• The --show-warnings command line flag has been removed in favor of the new options. (Put all warnings behind --show-warnings #1480)

Besides the added tests, ran grind compare-flutter-warnings to verify that the new warning code does not accidentally drop warnings on the floor or create new ones:

flutter-docs-current: Generating docs for library web_socket_channel.status from package:web_socket_channel/status.dart...
flutter-docs-current: found 887 warnings and 0 errors
flutter-docs-current: Documented 97 public libraries in 128.9 seconds
flutter-docs-current: Success! Docs generated into /tmp/flutterAPPAHT/dev/docs/doc/api
flutter-docs-current:
flutter-docs-current: Creating a custom index.html in dev/docs/doc/index.html
flutter-docs-current:
flutter-docs-current: Docs ready to go!
flutter-docs-original: Generating docs for library vector_math_geometry from package:vector_math/vector_math_geometry.dart...
flutter-docs-original: Generating docs for library vector_math_lists from package:vector_math/vector_math_lists.dart...
flutter-docs-original:   warning: unresolved doc reference [vectorLength], from vector_math_lists.ScalarListView.ScalarListView: (file:///tmp/pubcacheLWEWFN/hosted/pub.dartlang.org/vector_math-2.0.8/lib/src/vector_math_lists/scalar_list_view.dart:29:3)
flutter-docs-original:   warning: unresolved doc reference [vectorLength], from vector_math_lists.ScalarListView.fromList: (file:///tmp/pubcacheLWEWFN/hosted/pub.dartlang.org/vector_math-2.0.8/lib/src/vector_math_lists/scalar_list_view.dart:38:18)
flutter-docs-original: Generating docs for library vector_math_operations from package:vector_math/vector_math_operations.dart...
flutter-docs-original: Generating docs for library web_socket_channel from package:web_socket_channel/web_socket_channel.dart...
flutter-docs-original: Generating docs for library web_socket_channel.io from package:web_socket_channel/io.dart...
flutter-docs-original:   warning: web_socket_channel.io has no library level documentation comments, from web_socket_channel.io: (file:///tmp/pubcacheLWEWFN/hosted/pub.dartlang.org/web_socket_channel-1.0.9/lib/io.dart:5:9)
flutter-docs-original: Generating docs for library web_socket_channel.status from package:web_socket_channel/status.dart...
flutter-docs-original: found 887 warnings and 0 errors
flutter-docs-original: Documented 97 public libraries in 131.0 seconds
flutter-docs-original: Success! Docs generated into /tmp/dartdoc-comparison-flutterIYMRUF/dev/docs/doc/api
flutter-docs-original:
flutter-docs-original: Creating a custom index.html in dev/docs/doc/index.html
flutter-docs-original:
flutter-docs-original: Docs ready to go!
*** Flutter repo : No difference in warning output from master to HEAD (887 warnings found)

[coveralls]

Coverage increased (+0.2%) to 93.697% when pulling 0328e04 on warning-control into ea0392e on master.

[gspencergoog]

This looks great!

The only thing I don't really like is the "warning" terminology itself. It's confusing that something that is an error is referred to as a warning in so many places: there are "error warnings" and "warning warnings". I would have used the word "logging" or "feedback", or something else that doesn't indicate the severity by its name. That could be fixed in another renaming PR though.

[jcollins-g]

Yes, I found myself cringing a bit at that too. I'll see what I can do in a followup.