cd a
! go build
stderr '^ambiguous import: found package image in multiple modules:\s+image\s+.+\s.+image.+\s$'


cd ../b
! go build -mod=vendor
stderr '^main.go:4:5: ambiguous import: found package image in multiple directories:\s+.+image\s+.+image\s+$'

cd ../c
! go build -mod=vendor
stderr 'main.go:4:5: package p is not in std'

-- a/go.mod --
module image

-- a/main.go --
package main

func main() {
    println("hello world!")
}

-- b/go.mod --
module test

-- b/vendor/image/b.go --
package image
func Add(a, b int) int {
    return a + b
}

-- b/main.go --
package main

import (
    "image"
)

func main() {
    println(image.Add(1,1))
}

-- c/go.mod --
module test

-- c/main.go --
package main

import (
    "p"
)

func main() {
    println(p.Add(1,1))
}