(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.
The only case where it *could* make it into that metadata object is where the user's `metadata.json` (in the module root) contains that key.  While it's possible (even likely) that *if* it is present, it was put there by copying the build's `metadata.json` into the module root, that file is presently considered to be user-provided and -owned and will (soon?) be the exclusive unmodified source of module metadata truth.

To my understanding, the module docs overstate the importance of removing those two fields – I am not aware of any reason that modules containing those keys in their metadata should not be installable and publishable (in fact, the Forge is primarily composed of such modules).