Move Guile modules under modules/ .

This patch moves Guile modules shipped as a Guix channel under the
modules directory.  The rationale behind this is that we are probably
adding unwanted files into Guile's load path, such as the news file and
possible scripts that could be added to the channel repository in the future.

* guix-channel (channel): Set directory field.

* nongnu/*: Move under modules/nongnu.

* nonguix/*: Move under modules/nonguix.

.guix-channel

@@ -4,4 +4,5 @@
 (channel
  (version 0)
  (news-file "news.txt")
+ (directory "modules")
  (url "https://gitlab.com/nonguix/nonguix")) ; the primary URL