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