Add source location and checksum in generated JSON data.
Add source location and checksum in generated JSON data.

Fixes #804

gmalecha/opam-coq-archive,coq/opam-coq-archive,mattam82/coq-opam-archive,mattam82/coq-opam-archive,gmalecha/opam-coq-archive,mattam82/coq-opam-archive,gmalecha/opam-coq-archive