moq generates mocks with internal package imports

Nobody has claimed this yet.

In some situations, moq generates mocks that try to import internal 3rd party packages. Minimal example below.

example.go
```go
package app

import (
"context"

pubsub_api "cloud.google.com/go/pubsub"
)

type IPubSubTopic interface {
Exists(ctx context.Context) (bool, error)
Publish(ctx context.Context, msg *pubsub_api.Message) *pubsub_api.PublishResult
}
```
Here pubsub_api is an explicit alias to the public package.

Generate mock:
```
moq -out example_mock.go . IPubSubTopic
```

example_mock.go
```go
// Code generated by moq; DO NOT EDIT.
// github.com/matryer/moq

package app

import (
"cloud.google.com/go/internal/pubsub"
"context"
"sync"
)

// Ensure, that IPubSubTopicMock does implement IPubSubTopic.
// If this is not the case, regenerate this file with moq.
var _ IPubSubTopic = &IPubSubTopicMock{}
```
But the mock imports the internal pubsub package instead. I couldn't find any way to prevent it.

go version go1.15.6 linux/amd64
moq version: v0.2.3

Start by reproducing the issue with the provided example.go interface and the `moq -out example_mock.go . IPubSubTopic` command, then inspect the generated example_mock.go imports. Done means the generated mock preserves the public cloud.google.com/go/pubsub import and no longer references the internal package; verify the result with the example.

Written by the indexing model from the issue text.