In some cases, copying all variables failed when using tags to copy

[xichencx]

Reproducible Example

func TestName(t *testing.T) {
t.Run("validate error flag name", func(t *testing.T) {
type SrcTags struct {
field1 string
Field2 string
}

	type DestTags struct {
		Field1 string `copier:"field1"`
		Field2 string `copier:"Field2"`
	}

	dst := &DestTags{}
	src := &SrcTags{
		field1: "Field1->Field1",
		Field2: "Field2->Field2",
	}
	err := copier.Copy(dst, src)

	t.Log(dst)
	if err == nil {
		t.Fatal("must validate error")
	}
})

}

Description

src structure中存在私有变量，dst structure中使用copier标签进行拷贝会导致src structure中其他变量拷贝失败

[XiXiangFiles]

I don't believe this is a bug. The private field I expect should not be copied, and an error should be returned. What are your expectations regarding this?

[xichencx]

I don't believe this is a bug. The private field I expect should not be copied, and an error should be returned. What are your expectations regarding this?

如果按照上述说法，那我这种方式去进行copy的时候会成功

`func TestName(t *testing.T) {
t.Run("validate error flag name", func(t *testing.T) {
type SrcTags struct {
Field1 string
field2 string
}

	type DestTags struct {
		Field1 string `copier:"Field1"`
		Field2 string `copier:"Field2"`
	}

	dst := &DestTags{}
	src := &SrcTags{
		Field1: "Field1->Field1",
		field2: "Field2->Field2",
	}
	err := copier.Copy(dst, src)

	t.Log(dst)
	if err == nil {
		t.Fatal("must validate error")
	}
})

}`

[XiXiangFiles]

@xichencx
here I tried your codebase. it'll raise an error, is it in your expectation? here