Path names in import/export statements are relative to the file they are in.

This fixes the logic and unblocks using the messages test suite on Android,
which has cases importing files from mjsunit, which import more files from
there.

Bug: chromium:866862
Change-Id: I8d2ff645f69b67fbdaf4a622d06308e55298b0ce
Reviewed-on: https://chromium-review.googlesource.com/1206570Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55643}