[analyzer plugins] Plugin-reported diagnostics should be ignorable inline

[srawlins]

The design we've roughly landed on is that // ignore: and // ignore_for_file: comments will supported for plugin-defined diagnostics (lints and warnings) via a plugin-namespaced name:

// ignore: foo.bar

This will direct the analyzer plugin tooling to ignore a report of the bar diagnostic from the foo plugin.

Notice the period (.) delimiter. We discussed also the : delimiter, but that looks confusing with ignore:, especially since there does not need to be whitespace after ignore: (e.g. // ignore:foo:bar). We also considered /, which I believe tslint uses, something like // ignore: foo/bar. We decided that the period looks the most "darty," being reminiscent of property access, like "the bar diagnostic of the foo plugin."

[srawlins]

Two updates:

• I realized that internally at Google we can name packages with period separators (package:foo.bar.baz). So I think we should change the delimiter to the alternate we considered, /.
• This proposal does not yet address the type=lint|warning bit. If a file contains // ignore_for_file: type=warning, and an analyzer plugin produces a warning in that file, should it be ignored? I think yes. If it wasn't ignored, it would defeat the motivation for type=, which is to let code-generators generate code that complies with some unknown lint rule set.