Support caching/freezing pkg_resources entry_points

On some devices with sd card storage, `pkg_resources` is *very slow* since the underlying filesystem is very slow and pkg_resources does a file scan through all installed packages when you import it.

There should be a way to cache the results of this process in a single call so that we don't need to load `pkg_resources` every time a coretools based script starts and we know that the list of packages hasn't changed.