Proper detection of the LICENSE files

Currently, we ignore three kinds of files:
1. Files ending with `.license`.
2. Files with literal file name of `LICENSE.txt`.
3. Files in the `LICENSES` folder.

We should check what the spec says about this, and what files should actually be ignored.

See the code near this line: https://github.com/ForNeVeR/dotnet-reuse/blob/be3dd1818b0ea88685dae07cd847eab31fe6da46/FVNever.Reuse/ReuseDirectory.cs#L72

Also, look for the number of this issue in the project code base.