downloadDeps should detect infinite recursion

[thehowl]

A package with these settings:

module gno.land/tests/replaceavl

replace (
	"gno.land/p/demo/avl" => "gno.land/r/sys/users"
)

package main

import (
	avl "gno.land/p/demo/avl"
)

var foo = avl.RegisterUser

Hangs, because replaceavl imports r/sys/users, which in turn wants to import avl but it gets replaced with itself.

We should be able to detect this.

cc @n0izn0iz