Handle relative import paths.

[melindalu]

We ran into a problem with imports using relative paths to reference packages in the same repository of the form "../othersubpackage". @yugui @pmbethe09 does this look reasonable?

[bazel-io]

Can one of the admins verify this patch?

[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 you signed the CLA as a corporation, please let us know the company's name.

[melindalu]

I signed it!

[googlebot]

CLAs look good, thanks!

[yugui]

@melindalu
LGTM, but could you name the importpath lib/relativeimporter or just lib/relative instead oflib/relative_importer to avoid underscore character?

Jenkins, test this please.

[melindalu]

@yugui good point, thanks — changed.

[pmbethe09]

LGTM