go mod tidy repro case

go.mod

module example.com/foo

go 1.12

require (
        cloud.google.com/go v0.34.0
        github.com/google/martian v2.1.0+incompatible // indirect
        github.com/googleapis/gax-go v0.0.0-20170321005343-9af46dd5a171 // indirect
        github.com/mattn/go-sqlite3 v1.2.1-0.20170407154627-cf7286f069c3
        github.com/philhofer/fwd v1.0.0 // indirect
        github.com/tinylib/msgp v1.1.0 // indirect
        golang.org/x/net v0.0.0-20190311183353-d8887717615a
        golang.org/x/oauth2 v0.0.0-20190226205417-e64efc72b421
        golang.org/x/time v0.0.0-20170424234030-8be79e1e0910 // indirect
        google.golang.org/api v0.3.0
        zombiezen.com/go/capnproto2 v2.16.0+incompatible
        zombiezen.com/go/log v0.0.0-20170324213728-b02efba1fddb
)

main.go

package main

import (
        _ "database/sql"
        _ "errors"
        _ "flag"
        _ "fmt"
        _ "net"
        _ "os"
        _ "sort"
        _ "sync"
        _ "time"

        _ "cloud.google.com/go/storage"
        _ "cloud.google.com/go/trace"
        _ "github.com/mattn/go-sqlite3"
        _ "golang.org/x/net/context"
        _ "golang.org/x/oauth2/google"
        _ "google.golang.org/api/option"
        _ "zombiezen.com/go/capnproto2"
        _ "zombiezen.com/go/capnproto2/rpc"
        _ "zombiezen.com/go/capnproto2/server"
        _ "zombiezen.com/go/capnproto2/std/capnp/rpc"
        _ "zombiezen.com/go/log"
)

func main() {
}