Use SPDX license identifiers in the default bucket

[Calinou]

This leads to more consistent license properties and allows for easier automated parsing. I left the custom URLs in packages that use custom licenses or whose license isn't clearly defined.

See the SPDX License List for more information.

[r15ch13]

Please remove the changes from test fixtures broken_wget.json, invalid_wget.json and wget.json.
These files don't need a correct license identifier.

[Calinou]

I updated the pull request, please check it again.

[rasa]

I updated the wiki with:

• license: The software license for the program. For well-known licenses, please use the identifier found at https://spdx.org/licenses/ For other licenses, use the URL of the license, if available. Otherwise, use “Freeware”, “Shareware”, or “Custom”, as appropriate.

Is that acceptable?

[Calinou]

@rasa Sounds good to me. 😃