This is the first step in a two-step process of moving the package preparation step from a recipe in chrome_infra to a dart script in the flutter repo. This will make it easier to make changes to the process. The second step is to change the infra recipe to call this script.

In addition, I added a step to the packaging process to run flutter create for each type of template so that any pub dependencies of the templates get added to the cache that gets packaged (and thus users can run flutter create --offline and have it work).

Note that the actual packaging into a "tar" or "zip" file now happens here, so a developer could actually run this script on their machine to create a package.