Add support for parsing more than one package

[yash-nisar]

The current implementation only supports parsing a single package in the SPDX document. There can be one or more packages as mentioned here. We should add support for parsing more than one package.

[ro22110]

Not sure this issue has been resolved or I am doing something wrong, I just couldn't get it to work with multiple packages, I'am always getting the error:

spdx.parsers.builderexceptions.OrderError: File::SPDXID

[yogini-ghode]

@licquia , we are still facing same issue with multiple packages.
"spdx.parsers.builderexceptions.OrderError: File::SPDXID"
Can you please let us know how we can get over it?