We thought it would be OK to call the create command offline-only, but it turns out that isn't a great idea because not all of the create template dependencies are cached by running flutter_tool.
This PR makes it go online by default when running pub get after creating a project, and adds an "--offline" flag to the create command so that offline users can use it offline if their cache is warm.
In order to allow offline usage of the create command we need to be able to tell pub not to try and contact the server to see if new versions are available. Since all the versions that create could want are pinned, it shouldn't need to check for new versions.
Allows the user to specify the kind of project to create. The default is 'app'. Other choices are 'plugin' (the old '--plugin' behavior), and 'package'.
A Flutter 'package' is a Dart package that depends on Flutter, but does not contain native code.
Fixes#10377.
