Split cloud container to its own package

[cheungpat]

I would like to suggest we split the cloud container to its own package because the cloud container may (or will) depend on packages that are not necessary on the browser side. For example, argument parsing and structural logging.

For the time being, we can split the cloud container into its subpackage like skygear-cloud, then have skygear package depends on skygear-cloud, in order to maintain compatibility. We can then figure out how to remove the cloud part from skygear in the long run.