Split out the Package type into a first class Kubernetes resource. Before this change, packages were implicitly tied to functions.
This wasn't ideal because:
* Functions will need to share packages
* A package storage system may be more generally useful than just
functions (for example, for storing static assets)
This change does the following:
* Updates the fission and tpr types to add a new Package and PackageSpec. It also creates a PackageRef type, and a FunctionPackageRef type. The PackageRef simply references a package, but the FunctionPackageRef includes the name of a function within the package. This allows us to share packages between different functions.
* Updates fetcher and other components for first-class packages
* Allows customization of fetcher image pull policy in the helm charts
