Consider adding an auto-vivifying PackageMeta type based on package_config.json

[jcollins-g]

Instead of relying wholly on pubspec.yaml for package metadata, perhaps dartdoc should autovivify packages with their names, based on the package_config.json. This would be more in line with how the compiler/analyzer work, and make dartdoc more easily usable in situations where Dart is used without pub, e.g. in large company-wide mono-repos and similar.