emacs: Add "Source" field to 'guix-info' buffers.

Suggested by Ludovic Courtès.

* emacs/guix-info.el (guix-info-insert-methods, guix-info-displayed-params):
  Add 'source' parameter.
  (guix-package-info-source): New face.
  (guix-package-source): New button type.
  (guix-package-info-auto-find-source, guix-package-info-auto-download-source,
  guix-package-info-download-buffer): New variables.
  (guix-package-info-show-source, guix-package-info-insert-source-url,
  guix-package-info-insert-source, guix-package-info-download-source,
  guix-package-info-redisplay-after-download): New procedures.
* emacs/guix-base.el (guix-param-titles): Add 'source' parameter.
  (guix-operation-prompt): Add 'prompt' argument.
  (guix-after-source-download-hook): New variable.
  (guix-package-source-path, guix-package-source-build-derivation): New
  procedures.
* emacs/guix-main.scm (%package-param-alist): Add 'source' parameter.
  (package-source-names, package-source-derivation->store-path,
  package-source-path, package-source-build-derivation): New procedures.