我使用命令go get github.com/gorilla/mux。我使用Golang制作了http服务器，并运行了以下程序：

package main

import (
    "fmt"
    "html"
    "log"
    "net/http"

    "github.com/gorilla/mux"
)

func main() {

    router := mux.NewRouter().StrictSlash(true)
    router.HandleFunc("/", Index)
    log.Fatal(http.ListenAndServe(":8080", router))
}

func Index(w http.ResponseWriter, r *http.Request) {
    fmt.Fprintf(w, "Hello, %q", html.EscapeString(r.URL.Path))

但我对此错误提出了抵触：

/usr/local/go/bin/go build -i [/Users/imac/go/src]
http.go:9:5: cannot find package "github.com/gorilla/mux" in any of:
    /usr/local/go/src/github.com/gorilla/mux (from $GOROOT)
    ($GOPATH not set)
Error: process exited with code 1.

我的Go环境在这里：

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/imac/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/v9/fkc_t97s5v1g9sr938zzvxvh0000gn/T/go-build096571864=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"

我为这个错误解决了一个星期，但是我找不到解决方法。请帮我。