Add support for alternative SPDX identifiers formats

[pombredanne]

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
# <https://www.gnu.org/philosophy/free-sw.en.html>.

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

[pombredanne]

And in https://pagure.io/fedora-rust/rust2rpm @ignatenkobrain has a mapping for Fedora to SPDX https://pagure.io/fedora-rust/rust2rpm/blob/master/f/rust2rpm/spdx_to_fedora.csv and more such as a Cargo->RPM version constraints converter https://pagure.io/fedora-rust/rust2rpm/blob/master/f/rust2rpm/metadata.py