Add source information for Guix backend

[Ambrevar]

Currently OSPM does not report source information of Guix packages.

Some Scheme code that may help:

(package-source $2)
(procedure-name (origin-method (package-source $2)))
(origin-uri (package-source $2))

,use (guix base32)
(bytevector->nix-base32-string (content-hash-value (origin-hash (package-source $2))))
(content-hash-algorithm (origin-hash (package-source $2)))

(origin-modules (package-source $2))



scheme@(guile-user)> (with-output-to-string (lambda () (write  (origin-snippet (package-source $32)))))
$35 = "(begin (substitute* \"package.lisp\" ((\":export\") \":export\\n   :+font-cache-filename+\")) #t)"

(git-reference-url (origin-uri (package-source $32)))
(git-reference-commit (origin-uri (package-source $32)))
(git-reference-recursive? (origin-uri (package-source $32)))