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.
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