Add support for SPDX license IDs in License Policies `policies.yml` file

[silverhook]

Currently to write policies.yml one needs to use ScanCode’s own license names, which, to be honest, are not as well known as SPDX IDs.

It would be great, if one could use SPDX license short names instead (or in addition).

[tdruez]

@silverhook Thanks for the suggestion, support added in #1714