guix-package: Fix handling of the PACKAGE:OUTPUT syntax.

* guix-package.in (guix-package)[find-package]: Return the correct NAME
  and SUB-DRV when NAME contains #\:.

* tests/guix-package.sh (profile): Add test.

guix-package.in

@@ -284,8 +284,8 @@ Report bugs to: ~a.~%") "@PACKAGE_BUGREPORT@"))
     (let*-values (((name sub-drv)
                    (match (string-rindex name #\:)
                      (#f    (values name "out"))
-                     (colon (values (substring name (+ 1 colon))
-                                    (substring name colon)))))
+                     (colon (values (substring name 0 colon)
+                                    (substring name (+ 1 colon))))))
                   ((name version)
                    (package-name->name+version name)))
       (match (find-packages-by-name name version)

tests/guix-package.sh

@@ -40,4 +40,7 @@ guix-package -b -p "$profile" -r "guile-bootstrap-2.0"
 test -L "$profile-3-link"
 test -f "$profile/bin/make" && ! test -f "$profile/bin/guile"
 
+# Make sure the `:' syntax works.
+guix-package -b -i "libsigsegv:lib" -n
+
 rm "$profile" "$profile-"[0-9]*