When executing a script that does import("foo"), "foo" should be
relative to the script's directory, not relative to the current working
directory of the d8 process.

R=gsathya@chromium.org

Bug: v8:5785
Change-Id: Id6ceccb242905bd6b54b07038ece60b93d92e4a0
Reviewed-on: https://chromium-review.googlesource.com/546375
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46185}