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

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

Start by reading add_licenses.py and the documented wget command for licenses.json. Trace where the script currently expects that file, then verify that running add_licenses.py no longer requires a separate download or a manually placed licenses.json file.

Written by the indexing model from the issue text.