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
Previously even when installing dependencies from a lockfile,
jsonnet-bundler would attempt to resolve versions of transitive
dependencies to their latest floating version. Instead when a lock-file
is specified jsonnet-bundler should just install the specified versions.
