reflect: StructOf returns types with unpopulated StructField.PkgPath

At tip, the following:
```go
t := reflect.StructOf([]reflect.StructField{{
	Name:    "height",
	PkgPath: "path/to/pkg",
	Type:    reflect.TypeOf(float64(0)),
}})
fmt.Println(t.Field(0).PkgPath)
```
This currently prints nothing. I expect it to print `path/to/pkg` otherwise logic to detect unexported fields doesn't work properly.