project_sdk: Rename FindSourceRoot to FindRepoRoot.

This utility method had a confusing name, especially in a world with
workspaces. So... rename it.

BUG=None
TEST=lint + unittests

Change-Id: I4e2ee78f25566d2772450bd522fafb3294869806
Reviewed-on: https://chromium-review.googlesource.com/262436
Trybot-Ready: Don Garrett <dgarrett@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: David Pursell <dpursell@chromium.org>
Commit-Queue: Don Garrett <dgarrett@chromium.org>
3 files changed