为什么Go编译器找不到我使用的库？

I have a simple Go program in directory x:

package main

import (
        "log"
        "os"

        "github.com/yanzay/tbot"
)

func main() {
        bot, err := tbot.NewServer(os.Getenv("TELEGRAM_TOKEN"))
        if err != nil {
                log.Fatal(err)
        }
        bot.Handle("/answer", "42")
        bot.ListenAndServe()
}

x/pkg/windows_386/github.com/yanzay contains the file tbot.a with the tbot library used in my program. x/src/github.com/yanzay/tbot contains the source code of that library.

When I run go build main.go, I get this output:

# command-line-arguments
.\main.go:12:21: undefined: tbot.NewServer

How can I make sure that Go compiler finds the tbot package in the x/pkg or x/src?

undefined: tbot.NewServer means Go finds the tbot library but doesn't find the NewServer function.

There's no such function in this library.