discovery: create proxy for testing; add .DS_Store to .gitignore

Implemented a proxy that runs on localhost:8080 following the Module
proxy protocol.

It supports the modules and versions:

- my/module (v1.0.0, v1.1.0, v1.1.1)
- my/module/v2 (v2.0.0)

A GET request can be made to:

- GET $GOPROXY/<module>/@v/list
- GET $GOPROXY/<module>/@v/<version>.info
- GET $GOPROXY/<module>/@v/<version>.mod
- GET $GOPROXY/<module>/@v/<version>.zip

All the zip files contain only an empty README.md.

Change-Id: If7d547eb94a9a24608fafccdfd39330da9849a0e
Reviewed-on: https://team-review.git.corp.google.com/c/412883
Reviewed-by: Andrew Bonventre <andybons@google.com>

.gitignore

@@ -35,3 +35,6 @@ override.tf
 override.tf.json
 *_override.tf
 *_override.tf.json
+
+### macOS ###
+.DS_Store

internal/testdata/modproxy/main.go

@@ -0,0 +1,18 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"log"
+	"net/http"
+)
+
+const proxyURL = "./proxy"
+
+func main() {
+	http.Handle("/", http.FileServer(http.Dir(proxyURL)))
+
+	log.Fatal(http.ListenAndServe(":8080", nil))
+}

internal/testdata/modproxy/proxy/my/module/@v/list

@@ -0,0 +1,3 @@
+v1.0.0
+v1.1.0
+v1.0.1

internal/testdata/modproxy/proxy/my/module/@v/v1.0.0.info

@@ -0,0 +1,4 @@
+{
+    "Version": "v1.0.0",
+    "Time": "2019-01-30T00:00:00Z"
+}

internal/testdata/modproxy/proxy/my/module/@v/v1.0.0.mod

@@ -0,0 +1 @@
+module my/module

internal/testdata/modproxy/proxy/my/module/@v/v1.1.0.info

@@ -0,0 +1,4 @@
+{
+    "Version": "v1.1.0",
+    "Time": "2019-02-01T00:00:00Z"
+}

internal/testdata/modproxy/proxy/my/module/@v/v1.1.0.mod

@@ -0,0 +1 @@
+module my/module

internal/testdata/modproxy/proxy/my/module/@v/v1.1.1.info

@@ -0,0 +1,4 @@
+{
+    "Version": "v1.1.1",
+    "Time": "2019-02-06T00:00:00Z"
+}

internal/testdata/modproxy/proxy/my/module/@v/v1.1.1.mod

@@ -0,0 +1 @@
+module my/module

internal/testdata/modproxy/proxy/my/module/v2/@v/list

@@ -0,0 +1 @@
+v2.0.0

internal/testdata/modproxy/proxy/my/module/v2/@v/v2.0.0.info

@@ -0,0 +1,6 @@
+{
+    "Version": "v2.0.0",
+    "Name": "my/module/v2",
+    "Short": "v0.8.0",
+    "Time": "2019-02-08T00:41:07Z"
+}

internal/testdata/modproxy/proxy/my/module/v2/@v/v2.0.0.mod

@@ -0,0 +1 @@
+module my/module/v2