wplua: enable the use of require() to load lua libraries from scripts/lib

tests/wplua/scripts/lib/testlib.lua

@@ -0,0 +1,9 @@
+local testlib = {}
+
+function testlib.test_add_ten (x)
+  return x + 10
+end
+
+Log.info("in testlib")
+
+return testlib