Also change fetch_deps.py to no longer be a no-op and rename
Main function for importing from other scripts.

R=machenbach@chromium.org

Bug: v8:6105
Change-Id: I067a212827316248f60e97ff27e9bb2dc20addfd
Reviewed-on: https://chromium-review.googlesource.com/860007Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50522}