module example.com/a

go 1.15

require example.com/b v0.1.0
-- a/a.go --
package a
-- a/a_test.go --
package a

import (
	"testing"

	_ "example.com/b"
)

func TestUsingB(t *testing.T) {
	// …
}
-- b1/go.mod --
module example.com/b

go 1.15

require example.com/c v0.1.0
-- b1/b.go --
package b
-- b1/b_test.go --
package b