Fix Go tests to run in module mode

Stop using relative imports and add a go.mod file.

Tested against Go 1.6 through Go pre-1.13, and gccgo.

Examples/go/pointer/runme.go

@@ -1,8 +1,9 @@
 package main
 
 import (
-	. "./example"
 	"fmt"
+
+	. "example"
 )
 
 func main() {