Add option to verify that contents of the license file is valid

[sorairolake]

I think it would be useful if reuse lint has an option to verify that contents of the license file is valid.

This means that an error will be returned in the following cases:

• GPL-3.0-or-later.txt is an empty license file.
• The contents of Apache-2.0.txt is MPL-2.0.

However, I think this feature should be an option since it's not always possible to correctly guess the contents of the license file.

[silverhook]

I think this is a great idea!

Just yesterday I ran into a surprise whereupon reuse download --all, I ended up with two empty license files in the LICENSES/ folder.

However, I think this feature should be an option since it's not always possible to correctly guess the contents of the license file.

I’ve half a mind that it would make sense to enable it by default.

Any licenses that do not start with LicenseRef- are by definition part of the SPDX License List and correspond to the canonical plaintext versions from https://github.com/spdx/license-list-data/tree/main/text. So testing that (or with more work even against the templates) should be doable, I think.