create-spdx: transform license list into a dict for faster lookups

create-spdx: transform license list into a dict for faster lookups

spdx-licenses.json contains an array of licenses objects. As we'll be
searching it often, convert that to a dictionary when we parse it.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>