(PUP-1977) Update `puppet module generate` to create `metadata.json`

Prior to this commit, running `puppet module generate` would create a Modulefile with the appropriate metadata, from which a metadata.json file would be generated when you ran `puppet module build`.  Since we're migrating away from the Modulefile as the canonical source of metadata (as per PE-3117), this commit will instead generate an equivalent metadata.json file when the module is created.