Add go environment vendor directory support (#781)

* Add go environment vendor directory support
* Use symbolink instead of copying files to gopath
* Add go vendor example package & test

examples/go/vendor-example/main.go

@@ -0,0 +1,12 @@
+package main
+
+import (
+	"go/vendordir/test"
+	"net/http"
+)
+
+// Handler is the entry point for this fission function
+func Handler(w http.ResponseWriter, r *http.Request) {
+	msg := test.Test()
+	w.Write([]byte(msg))
+}

examples/go/vendor-example/vendor/go/vendordir/test/test.go

@@ -0,0 +1,5 @@
+package test
+
+func Test() string {
+    return "vendor hello world"
+}