package:build_runner v2.6.0

• Remove unused deps: analyzer, build_resolvers, collection, http,
  js, meta, package_config, pool, web, yaml.
• Remove unused dev_deps: http, package_config, web.
• Testing: read build configs using AssetReader so they're easier to test.
• Use build 3.0.0.
• Use build_resolvers 3.0.0.