Dear Guix,

I would like to expand the Cypher back-end and in the long run add a
SPARQL graph back-end to GNU Guix.  For this, I will need to have access
to the package records inside the emit-* functions.

This patch makes this change by essentially changing the "label"
parameter of the emit-* functions passed as "(node-label head)" into a
"node" parameter, passed as "head".

The rest of the patch adapts the current emit-* functions to this
change.

I tested the Graphviz, D3js, and Cypher back-ends, and all seem to work
as before.

Is it OK to apply this change?

Kind regards,
Roel Janssen