goose: use marker functions instead of comments

To avoid making this CL too large, I did not migrate the existing goose
comments through the repository.  This will be addressed in a subsequent
CL.

Reviewed-by: Tuo Shan <shantuo@google.com>

internal/goose/testdata/PkgImport/bar/bar.go

@@ -2,7 +2,6 @@ package bar
 
 type Bar int
 
-//goose:provide Bar
 func ProvideBar() Bar {
 	return 1
 }