Allow enable on constructors like NewXXX

[pengubco]

I ran into some nil pointer bug which was caused by pattern like this.

func NewXXX(client Client, x X,  y Y) *A {
 return &A{
   x: x,   
   y: y 
 }
}  

The nil pointer was from code r.client.Foo(). That leads me to unparam which could've have reported I forgot to set client.
However, unparam recommended to rurn off -exported. I wonder whether we can turn on just those constructors like NewxXX.
How do you think?