Add unlicense support to add3prf.py

This adds the ability to detect and generate the correct METADATA file
for unlicense licensed crates.

Bug: 317361040
Test: atest --host add3prf_test; Ran against https://crates.io/crates/aho-corasick
Change-Id: Icbed2fbab7368f5faef2194a4836c369b038979c
2 files changed