Add an enforce-lockfile command line option

[Wdestroier]

Hi!

When compiling an app to production it's important to know how it will behave. Dart has a pubspec.lock file with the exact version of each dependency. pubspec.yaml may resolve different versions in local/development environment and later when the app is published. Apparently webdev doesn't have a --enforce-lockfile flag. This flag is documented in https://dart.dev/tools/pub/packages#get-dependencies-for-production.

How can we make webdev use the pubspec.lock file?