Get license from GitHub

[HebaruSan]

Motivation

The GitHub API provides license data:

https://api.github.com/repos/Starstrider42/Custom-Asteroids-Extras

  "license": {
    "key": "mit",
    "name": "MIT License",
    "spdx_id": "MIT",
    "url": "https://api.github.com/licenses/mit",
    "node_id": "MDc6TGljZW5zZTEz"
  },

But currently Netkan doesn't use this info.
We do use API license data from Curse and SpaceDock.

Changes

Now Netkan sets the API's license.spdx_id value to the module's license property.

Fixes #2662.