store-copy: 'read-reference-graph' returns a list of records.

store-copy: 'read-reference-graph' returns a list of records.

The previous implementation of 'read-reference-graph' was good enough
for many use cases, but it discarded the graph structure, which is
useful information in some cases.

* guix/build/store-copy.scm (<store-info>): New record type.
(read-reference-graph): Rewrite to return a list of <store-info>.
(closure-size, populate-store): Adjust accordingly.
* gnu/services/base.scm (references-file): Adjust accordingly.
* gnu/system/vm.scm (system-docker-image): Likewise.
* guix/scripts/pack.scm (squashfs-image, docker-image): Likewise.
* tests/gexp.scm ("gexp->derivation #:references-graphs"): Likewise.