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.

Doc/Manual/Go.html

@@ -266,6 +266,13 @@ swig -go -help
   ignored.</td>
 </tr>
 
+<tr>
+<td>-import-prefix &lt;prefix&gt;</td>
+<td>A prefix to add when turning a %import prefix in the SWIG
+  interface file into an import statement in the Go file.  For
+  example, with <code>-import-prefix mymodule</code>, a SWIG
+  interface file <code>%import mypackage</code> will become a Go
+  import statement <code>import "mymodule/mypackage"</code>.</td>
 </table>