Update add_licenses.py to query for SPDX licenses on its own

[danielmachlab]

Currently, running add_licenses.py requires an extra step to be run before it is executed:

wget https://github.com/spdx/license-list-data/raw/master/json/licenses.json

Also, add_licenses.py expects the licenses.json file to be in the correct place when it is executed, and it will fail if it is not found.

Instead, the call to download licenses.json from Github should be incorporated into the add_licenses.py script to remove the need for an extra step and room for user error

See PR review comment: https://github.com/bazelbuild/rules_license/pull/17#discussion_r789012979