Expose SPDX license key and is_exception in the index.json

[pombredanne]

In order to help validating SPDX license expressions, I would like to have these extra attributes:

1. spdx_license_key
2. other_spdx_license_keys
3. is_exception

This way I can create a list of license_expression.LicenseSymbol objects and validate easily. This is going to be useful for aboutcode-org/license-expression#56

[tdruez]

Deployed. See https://scancode-licensedb.aboutcode.org/index.json