Spec: Handle LGPL-3.0 requiring GPL-3.0

[mxmehl]

LGPL-3.0 mandates that the corresponding GPL-3.0 license text is available in the repo as well. This would break the current REUSE spec as it requires that no *unused** licenses are stored in the LICENSES/ directory.

Unless FSF and SPDX find a good solution to this, REUSE would have to add this exception to the spec and tool.

In the spec, change A Project MUST NOT include License Files for licenses under which none of the files in the Project are licensed. to A Project MUST NOT include License Files for licenses under which none of the files in the Project are licensed, unless it is one of the LGPL-3.0 license for which the corresponding GPL-3.0 license text MUST be included.

In the tool:

• If the tool downloads LGPL-3.0-*, download the corresponding GPL-3.0-*
• If linting, ignore the corresponding GPL-3.0 license text if LGPL-3.0 license found in code, unless there is actually code licensed under this GPL-3.0 license.

[mxmehl]

This is obsolete now as SPDX offers a combined license text and the tools downloads this.