descriptor templates under the import relation. I.e., we can now
say:
nix-instantiate outdir foo.nix
which will create descriptors for foo.nix and all imported packages
in outdir/.
files) are now referenced using their cryptographic hashes.
This ensures that if two package descriptors have the same contents,
then they describe the same package. This property is not as
trivial as it sounds: generally import relations cause this property
not to hold w.r.t. temporality. But since imports also use hashes
to reference other packages, equality follows by induction.
