go/types: use new importer.ForCompiler for gotype command

This will produce better error messages (position information)
for errors referring to imported objects.

Change-Id: I24646ae803e6b8f78e9240310a858d4095e9463d
Reviewed-on: https://go-review.googlesource.com/c/152538Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/gotype.go

@@ -297,7 +297,7 @@ func checkPkgFiles(files []*ast.File) {
 			}
 			report(err)
 		},
-		Importer: importer.For(*compiler, nil),
+		Importer: importer.ForCompiler(fset, *compiler, nil),
 		Sizes:    types.SizesFor(build.Default.Compiler, build.Default.GOARCH),
 	}