Goでパッケージが読み込めない

GMail APIでメール送信（Golang）こちらの記事にあるコードを使いメールアドレスの部分だけを変更して実行してみたところライブラリをいくつか読み込むことができずにビルドが失敗してしまいました。

Go
1package main
2
3import (
4    "fmt"
5    "log"
6    "strings"
7    "time"
8
9    "encoding/base64"
10    "golang.org/x/oauth2"
11    "golang.org/x/oauth2/google"
12    gmail "google.golang.org/api/gmail/v1"
13)
14
15func main() {
16
17    config := oauth2.Config{
18        ClientID:     "Your Client ID",
19        ClientSecret: "Your Client Secret",
20        Endpoint:     google.Endpoint,
21        RedirectURL:  "urn:ietf:wg:oauth:2.0:oob",
22        Scopes:       []string{"https://mail.google.com/"},
23    }
24
25    expiry, _ := time.Parse("2006-01-02", "2017-07-11")
26    token := oauth2.Token{
27        AccessToken:  "Your Access Token",
28        TokenType:    "Bearer",
29        RefreshToken: "Your Refresh Token",
30        Expiry:       expiry,
31    }
32
33    client := config.Client(oauth2.NoContext, &token)
34
35    srv, err := gmail.New(client)
36    if err != nil {
37        log.Fatalf("Unable to retrieve gmail Client %v", err)
38    }
39
40    temp := []byte("From: 'me'\r\n" +
41        "reply-to: reply-to@example.jp\r\n" +
42        "To: to@example.jp\r\n" +
43        "Subject: TestSubject\r\n" +
44        "\r\n" + "TestBody")
45
46    var message gmail.Message
47    message.Raw = base64.StdEncoding.EncodeToString(temp)
48    message.Raw = strings.Replace(message.Raw, "/", "_", -1)
49    message.Raw = strings.Replace(message.Raw, "+", "-", -1)
50    message.Raw = strings.Replace(message.Raw, "=", "", -1)
51
52    _, err = srv.Users.Messages.Send("me", &message).Do()
53    if err != nil {
54        fmt.Printf("%v", err)
55    }
56}

調べてみたのですが、何が問題なのかわかりませんでした。

Tarminal
1gmail.go:10:5: cannot find package "golang.org/x/oauth2" in any of:
2	/usr/local/opt/go/libexec/src/golang.org/x/oauth2 (from $GOROOT)
3	/Users/harumi_sagawa/code/golang/src/golang.org/x/oauth2 (from $GOPATH)
4gmail.go:11:5: cannot find package "golang.org/x/oauth2/google" in any of:
5	/usr/local/opt/go/libexec/src/golang.org/x/oauth2/google (from $GOROOT)
6	/Users/harumi_sagawa/code/golang/src/golang.org/x/oauth2/google (from $GOPATH)
7gmail.go:12:5: cannot find package "google.golang.org/api/gmail/v1" in any of:
8	/usr/local/opt/go/libexec/src/google.golang.org/api/gmail/v1 (from $GOROOT)
9	/Users/harumi_sagawa/code/golang/src/google.golang.org/api/gmail/v1 (from $GOPATH)