Golang - Why can't I import local package in GOPATH/src/project but can in home directory?

I have a project, its folder structure is like following:

    /project
        models/
            Product.go
        main.go

The content of main.go is:

package main

import (
    "./models"
	"fmt"
    "github.com/gin-gonic/gin"
)

func main() {
    r := gin.Default()
	fmt.Println(models.Product{})
    r.GET("/", func(c *gin.Context) {
	    c.String(200, "he")
	})

    r.Run(":3000")
}

The content of Product.go is:

package models

type Product struct {
    Name string
}

What I get from typing go env is:

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/Mac/go"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.5.3/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.5.3/libexec/pkg/tool/darwin_amd64"
GO15VENDOREXPERIMENT=""
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -    fmessage-length=0 -fno-common"
CXX="clang++"
CGO_ENABLED="1"

When the location of the project directory is $GOPATH/src/project, If I run go run main.go, what I get is this error message: ./main.go:: can't find import: "github.com/gin-gonic/gin".

But when the location of project directory is ~/project, go run main.go can work as expected.

I use go1.5.3.

Can anyone help me. Thanks.

Relative import paths are only allowed as a convenience, mostly for experimentation. They are not fully supported by go build and go install. If you want your package to work with the go tools, don't use relative imports. Structure your code as described in How to Write Go Code.

As you are using "github.com/gin-gonic/gin" in your code. It is an external import. So go compiler will try to find this package in your workspace. So you need manually download those package in your go workspace or build path
"go get github.com/gin-gonic/gin".