Migrate to package_config

[mit-mit]

The .packages file is being discontinued, so we should migrate to package_config.json (or rather its public API).

Some current uses:
https://cs.github.com/dart-lang/coverage?q=.packages

[liamappelbe]

Looks like we already support package_config.json, and just use .packages as a fallback (see _parsePackages in resolver.dart). There's a few tests and tools to migrate though.

[liamappelbe]

Users can pass .dart_tool/package_config.json or .packages to the --package flag in format_coverage. But it would probably make more sense to deprecate this flag and add a --package flag that takes the root directory of the package and uses findPackageConfig.

[liamappelbe]

Closing this because the migration is done. Follow up work to change the flags tracked here: #484