Add API to return licenses as a map in addition to list

[dwalluck]

I see that the license list is actually using a map internally, but the API returns a list Map::values(). Is it possible to add an API which also returns a Map with key String licenseId and value SpdxListedLicense? This would be more performant rather than having to recreate a map from the list.

[goneall]

Sounds like a useful API - if you'd like to create a PR, I can review merge. Otherwise, I'll take a look at this after implementing the SPDX 3.0 support.