x/tools/gopls: source.doc code action: missing NewT func that returns unexported type t

[adonovan]

The "Show documentation" code action has missing functions. Specifically, a function NewT that returns an instance of a type is grouped under that type as a "constructor" function. But if the type is unexported, neither the type nor its constructors are shown, even if they are exported.

Repro:

package a

type A int

func NewA() A { return 0 }

type a int

func Newa() a { return 0 }