[check-licenses] Refactor allowlist to nested subcommands

Refactor AllowlistCommand into a nested sub-commander that delegates to
AllowlistAddCommand. This moves the mandatory -bug and optional -desc
flags to the add sub-command, aligning with standard CLI syntax
(allowlist add -bug ...). Implemented UX error catching to detect flags
placed after positional arguments and print helpful instructions.
Updated usage strings, fix command, and unit tests.

Change-Id: I61b8b7a6228e81cb5aea2fd5c5a67ecd74a1af7d
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1602050
Commit-Queue: Jerry Belton <jcecil@google.com>
Reviewed-by: Jiaming Li <lijiaming@google.com>