(PUP-1976) Use metadata.json as canonical metadata source.

Prior to this commit, the Modulefile was used as the primary source of user-facing metadata for modules.  This file was executable Ruby, which was intended to describe a simple data structure.  This causes a few problems for us in the long-term, including making automated module builds a significantly more difficult proposition, and making changes to acceptable metadata a slow process (requiring new releases of Puppet).  In addition, the only piece of Puppet code that interacted directly with the Module file was the `module build` action – anywhere we actually need to know something about the module, we look it up from the metadata.json file.

This commit removes this level of indirection between user input and module data, making the metadata.json file a first-class citizen in Puppet modules.  It similarly deprecates the Modulefile as a supported source of module data.  Checksums have also been moved into their own file (per PE-3124), and `puppet module changes` has been updated to leverage the new file when available (per PE-3120).

For users building a module containing only a Modulefile, this will issue a warning that the Modulefile is deprecated, and output the corresponding metadata.json file.

For users building a module containing both a Modulefile and a metadata.json file, this will issue a warning that the Modulefile is deprecated and is being ignored.