Packages are unique anyways so it makes sense to use a map to avoid having
duplicates.
For compatibility reasons custom json (un)marshallers hide this change from the
end user
rewrites the installation of packages from scratch to solve several issues with
the existing implementation:
- does not need to choose between lockfile and jsonnetfile anymore. The
jsonnetfile what to be installed, while the lockfile also has versions and
checksums of all packages, even nested ones.
- the lockfile is regenerated on every run, preserving the locked values
- downloaded packages are hashed using sha256 to make sure we receive what we
expect. If files on the local disk are modified, they are downloaded again.
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
