This particular error wouldn't belong in the import parsing section. Firstly, the error is raised on the line with the import, not on the line with the package declaration. Secondly, it is not necessary that the offending token is a package declaration. The below snippet just as well triggers this error:

package main

import "fmt"

func main() {}

import "net/http"

Regarding the snippet you have shared, it raises the error "non-declaration statement outside function body" on the line with the duplicate package declaration. That is not handled as a part of the import parsing, and is a completely different error.

@snprajwal I found that duplicate package declaration will also add an error element to parser.errors, the error info like blow:

The error you mentioned "non-declaration statement outside function body" seem like another tool:
https://github.com/golang/go/blob/master/src/cmd/compile/README.md#:~:text=the%20second%20phase.-,Note,-that%20the%20go