(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.
So, this is here specifically for the case where someone has a `metadata.json` file in their repo that includes a `"checksums"` key?  I don't disagree that it doesn't belong there, but I'm a *little* bit squeamish about removing keys that were deliberately provided by the user...