gnu: go-github-com-lucasb-eyer-go-colorful: Move to golang-xyz.

* gnu/packages/golang.scm (go-github-com-lucasb-eyer-go-colorful): Move
from here ...
* gnu/packages/golang-xyz.scm: ... to here.

* gnu/packages/image-viewers.scm: Add golang-xyz.

Change-Id: I9bb92dc5e5b549e74f6354870a061d958ff34931

gnu/packages/golang-xyz.scm

@@ -9175,6 +9175,33 @@ Printf/Sprintf etc.")
     (native-inputs
      (list go-github-com-stretchr-testify))))
 
+(define-public go-github-com-lucasb-eyer-go-colorful
+  (package
+    (name "go-github-com-lucasb-eyer-go-colorful")
+    (version "1.2.0")
+    (source
+     (origin
+       (method git-fetch)
+       (uri (git-reference
+             (url "https://github.com/lucasb-eyer/go-colorful")
+             (commit (string-append "v" version))))
+       (file-name (git-file-name name version))
+       (sha256
+        (base32 "08c3fkf27r16izjjd4w94xd1z7w1r4mdalbl53ms2ka2j465s3qs"))))
+    (build-system go-build-system)
+    (arguments
+     (list
+      #:import-path "github.com/lucasb-eyer/go-colorful"))
+    (propagated-inputs
+     (list go-golang-org-x-image))
+    (home-page "https://github.com/lucasb-eyer/go-colorful")
+    (synopsis "Library for playing with colors in Go")
+    (description
+     "The colorful package provides a library for using colors in Go.
+It stores colors in RGB and provides methods for converting these to various
+color spaces.")
+    (license license:expat)))
+
 (define-public go-github-com-lyft-protoc-gen-star-v2
   (package
     (name "go-github-com-lyft-protoc-gen-star-v2")