Fix import paths that are in vendor

[kohkimakimoto]

At now, interfacer generates import paths like the following:

// Code generated by interfacer; DO NOT EDIT

package main

import (
        "database/sql"
    "<my_repository>/vendor/github.com/pkg/errors"
)

// ...

This PR fix it like the following:

// Code generated by interfacer; DO NOT EDIT

package main

import (
        "database/sql"
    "github.com/pkg/errors"
)

// ...

I referred to https://github.com/matryer/moq/pull/122 and https://github.com/matryer/moq/issues/18 for resolving this issue.