cmd/cue/cmd: convert package block comments in cue get go

Pass package comments through writeDoc.

Issue #173

Change-Id: Ib0438ed38012bcb15128d1315b654111285753d9
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/4140
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cmd/cue/cmd/get_go.go

@@ -458,9 +458,7 @@ func (e *extractor) extractPkg(root string, p *packages.Package) error {
 		fmt.Fprintln(w, "//cue:generate cue get go", args)
 		fmt.Fprintln(w)
 		if f.Doc != nil {
-			for _, c := range f.Doc.List {
-				fmt.Fprintln(w, c.Text)
-			}
+			writeDoc(w, f.Doc)
 		}
 		fmt.Fprintf(w, "package %s\n", p.Name)
 		fmt.Fprintln(w)

cmd/cue/cmd/testdata/code/go/pkg1/file1.go

@@ -12,6 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+/*
+  block comment
+*/
 package pkg1
 
 import (

cmd/cue/cmd/testdata/pkg/cuelang.org/go/cmd/cue/cmd/testdata/code/go/pkg1/file1_go_gen.cue

@@ -2,6 +2,7 @@
 
 //cue:generate cue get go cuelang.org/go/cmd/cue/cmd/testdata/code/go/pkg1
 
+// block comment
 package pkg1
 
 import (