Adding a dep, or updating a dependency version makes writes to the
jsonnet files.
We evaluate the changes on each of the files. An empty jsonnetfile.json
does not create a corresponding lockfile, as a missing lockfile is not
different from its previous (non existent).
So far, `pkg` and `pkg/jsonnetfile` had overlapping functionality when it came
to choosing and loading jsonnetfiles.
This fully switches to the separate package `pkg/jsonnetfile` that seems to be
created for exactly this purpose