Allow to run dep ensure in working directories which are symlinks

[mizuochik]

This PR is small enhancement and solves #740.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If your company signed a CLA, they designated a Point of Contact who decides which employees are authorized to participate. You may need to contact the Point of Contact for your company and ask to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the project maintainer to go/cla#troubleshoot.
• In order to pass this check, please resolve this problem and have the pull request author add another comment and the bot will run again.

[googlebot]

CLAs look good, thanks!

[mizuochik]

I signed it!

[ofpiyush]

@Keitax I tried your branch and it fails for the reverse case where the project root is symlink to a directory not in GOPATH

#641 already solves your issue. Can you try that and confirm?

[mizuochik]

@ofpiyush Thank you for your comment. I saw #641. It seems what I want and I close this PR.