(PUP-2936) Fix checksum during PMT build (stable branch)

Prior to this commit puppet module build was checksumming the source directory
and putting that in checksums.json.
This commit corrects that behavior so it checksums the built module instead.
Additionally, it removes the deprecated and confusing checksums field from
metadata.json because there is no reason to keep it around during a build.
Isn't the checksums field the old way of storing the module checksums before we had checksums.json? Is it really considered user provided information? In the module docs, it is stated that you must remove this field: http://docs.puppetlabs.com/puppet/latest/reference/modules_publishing.html .