add gvt importer integration test and update docs

Author: michael-go

cmd/dep/init.go

@@ -29,7 +29,7 @@ specified, use the current directory.
 When configuration for another dependency management tool is detected, it is
 imported into the initial manifest and lock. Use the -skip-tools flag to
 disable this behavior. The following external tools are supported:
-glide, godep, vndr, govend.
+glide, godep, vndr, govend, gvt.
 
 Any dependencies that are not constrained by external configuration use the
 GOPATH analysis below.

cmd/dep/testdata/harness_tests/init/gvt/case1/final/Gopkg.lock

@@ -0,0 +1,3 @@
+
+[[constraint]]
+  name = "github.com/sdboyer/deptestdos"

cmd/dep/testdata/harness_tests/init/gvt/case1/final/Gopkg.toml

@@ -0,0 +1,16 @@
+// Copyright 2017 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 (
+	"fmt"
+
+	"github.com/sdboyer/deptestdos"
+)
+
+func main() {
+	var x deptestdos.Bar
+	fmt.Println(x)
+}

cmd/dep/testdata/harness_tests/init/gvt/case1/initial/main.go

@@ -0,0 +1,17 @@
+{
+  "version": "0",
+  "dependencies": [
+    {
+      "importpath": "github.com/sdboyer/deptest",
+      "repository": "https://github.com/sdboyer/deptest",
+      "revision": "3f4c3bea144e112a69bbe5d8d01c1b09a544253f",
+      "branch": "master"
+    },
+    {
+      "importpath": "github.com/sdboyer/deptestdos",
+      "repository": "https://github.com/sdboyer/deptestdos",
+      "revision": "5c607206be5decd28e6263ffffdcee067266015eXXX",
+      "branch": "master"
+    }
+  ]
+}

cmd/dep/testdata/harness_tests/init/gvt/case1/initial/vendor/manifest

@@ -0,0 +1,13 @@
+{
+  "commands": [
+    ["init", "-no-examples"]
+  ],
+  "error-expected": "",
+  "gopath-initial": {
+    "github.com/sdboyer/deptest": "3f4c3bea144e112a69bbe5d8d01c1b09a544253f"
+  },
+  "vendor-final": [
+    "github.com/sdboyer/deptest",
+    "github.com/sdboyer/deptestdos"
+  ]
+}

cmd/dep/testdata/harness_tests/init/gvt/case1/testcase.json

@@ -209,7 +209,7 @@ about what's going on.
 During `dep init` configuration from other dependency managers is detected
 and imported, unless `-skip-tools` is specified.
 
-The following tools are supported: `glide`, `godep`, `vndr` and `govend`.
+The following tools are supported: `glide`, `godep`, `vndr`, `govend` and `gvt`.
 
 See [#186](https://github.com/golang/dep/issues/186#issuecomment-306363441) for
 how to add support for another tool.