Add support for alternative SPDX identifiers formats

Reported by @Conan-Kudo
See https://pagure.io/obs-packaging-scripts/blob/master/f/distgit-obsimport.py#_4-14
```
#!/usr/bin/env python3
# -*- coding: utf-8 -*-

# Copyright (C) 2015-2017 Datto, Inc.
# Copyright (C) 2020 Neal Gompa
#
# Fedora-License-Identifier: GPLv2+
# SPDX-2.0-License-Identifier: GPL-2.0+
# SPDX-3.0-License-Identifier: GPL-2.0-or-later
#
# This program is free software.
# For more information on the license, see COPYING.
# For more information on free software, see
# .
```

There are a few things to consider there:
- Three different prefixes for a license expression
- Three different license symbols lists, the later two being based on different versions of SPDX license lists
- Notice-like text that is not license-specific starting at `This program is free software. ...`

See also https://github.com/search?q=SPDX-3.0-License-Identifier&type=code