(PUP-2936) Fix checksum during PMT build

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.
(PUP-2936) Add warning for 'checksums'

This commit adds a warning to `puppet module build` when it finds a 'checksums'
field in metadata.json. This was done to make people aware of the fact the field
is deprecated and will not be used by the builder, although the installer/upgrader
and other module tools will use the field if checksums.json is not present.