Loading module multiple times is not idempotent

[icexin]

Code as follows

func TestDoubleLoadModule(t *testing.T) {
	loader := CreateLoaderFromSource(`{
		module.exports = "bar";
	}`, "")

	testLoad := func() {
		vm := New()
		vm.AddModule("foo", loader)
		v, err := vm.Require("foo", ".")
		if err != nil {
			t.Fatal(err)
		}
		s, _ := v.ToString()
		if s != "bar" {
			t.Errorf("expect bar, got %s", s)
		}
	}
	testLoad()
	testLoad()
}

Output of test

--- FAIL: TestDoubleLoadModule (0.00s)
    motto_test.go:85: expect bar, got [object Object]
FAIL

The reason is that multiple calls to ModuleLoader cause the source to be repeatedly assigned.

motto/module.go

Line 29 in c7fa5e7

source = "(function(module) {var require = module.require;var exports = module.exports;var __dirname = module.__dirname;" + source + "\n})"