cargo2ebuild.py: Generate ebuilds for all crates in a workspace

BUG=none
TEST=verify that scripts generates build for all crates in a
  workspace (for example, parking_lot).

Change-Id: I680e9e25fb6b62ea57d2fbda6a70e71d03fff92c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/3322714
Reviewed-by: Allen Webb <allenwebb@google.com>
Commit-Queue: Kristian Kristensen <hoegsberg@chromium.org>
Tested-by: Kristian Kristensen <hoegsberg@chromium.org>