Move versioned generators into their own package

pkg/kubectl has a lot of files. Move everything generator related
into pkg/kubectl/generate (generic) or pkg/kubectl/generate/versioned
(type specific).

Move the DescriberFn and GeneratorFn out of kubectl/cmd/util and into
the respective versioned packages, along with tests.

build/visible_to/BUILD

@@ -392,9 +392,7 @@ package_group(
     packages = [
         "//pkg/kubectl",
         "//pkg/kubectl/cmd",
-        "//pkg/kubectl/cmd/logs",
-        "//pkg/kubectl/cmd/portforward",
-        "//pkg/kubectl/cmd/rollingupdate",
+        "//pkg/kubectl/generate",
         "//pkg/kubectl/proxy",
     ],
 )