package:build_runner v2.7.1

• Bug fix: with build_runner serve, special handling of paths containing
  /packages/ was hiding actual folders called packages. Serve the actual
  folders first, before trying the package lookup.
• Remove undocumented / unused features: lib/build_script_generate.dart,
  bin/graph_inspector.dart, build_runner doctor and
  build_runner generate-build-script commands.
• Remove deps on pubspec_parse and timing.
• Use build 3.1.0.