pkg/tool: generate package documentation

Fixes #39

Change-Id: I3acc6bd67a8012dd8a33819d153ee5c97342faa5
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/1926
Reviewed-by: Marcel van Lohuizen <mpvl@google.com>
diff --git a/pkg/tool/file/file.go b/pkg/tool/file/file.go
index e5673cc..29bf6d9 100644
--- a/pkg/tool/file/file.go
+++ b/pkg/tool/file/file.go
@@ -12,9 +12,10 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// Package file provides file operations for cue tasks.
 package file
 
+//go:generate go run gen.go
+
 import (
 	"io/ioutil"
 	"os"