Vendoring test deps fails when using external test package

[ivy]

Using this repo as an example, executing manul -I -t results in no dependencies being installed:

2018-02-01 22:17:41  [INFO]: all dependencies already vendored

I suspect this is because of the use of an external package for tests which uses the key XTestImports instead of TestImports in the output of go list -e -json github.com/ivy/manul-test-deps-bug:

{
	"Dir": "/Users/ivy/src/github.com/ivy/manul-test-deps-bug",
	"ImportPath": "github.com/ivy/manul-test-deps-bug",
	"Name": "main",
	"Target": "/Users/ivy/bin/manul-test-deps-bug",
	"Stale": true,
	"StaleReason": "cannot stat install target",
	"Root": "/Users/ivy",
	"GoFiles": [
		"main.go"
	],
	"Imports": [
		"fmt"
	],
	"Deps": [
		"errors",
		"fmt",
		"internal/cpu",
		"internal/poll",
		"internal/race",
		"io",
		"math",
		"os",
		"reflect",
		"runtime",
		"runtime/internal/atomic",
		"runtime/internal/sys",
		"strconv",
		"sync",
		"sync/atomic",
		"syscall",
		"time",
		"unicode",
		"unicode/utf8",
		"unsafe"
	],
	"XTestGoFiles": [
		"main_test.go"
	],
	"XTestImports": [
		"github.com/stretchr/testify/assert",
		"testing"
	]
}