chore: make yuzu REUSE compliant

[REUSE] is a specification that aims at making file copyright
information consistent, so that it can be both human and machine
readable. It basically requires that all files have a header containing
copyright and licensing information. When this isn't possible, like
when dealing with binary assets, generated files or embedded third-party
dependencies, it is permitted to insert copyright information in the
`.reuse/dep5` file.

Oh, and it also requires that all the licenses used in the project are
present in the `LICENSES` folder, that's why the diff is so huge.
This can be done automatically with `reuse download --all`.

The `reuse` tool also contains a handy subcommand that analyzes the
project and tells whether or not the project is (still) compliant,
`reuse lint`.

Following REUSE has a few advantages over the current approach:

- Copyright information is easy to access for users / downstream
- Files like `dist/license.md` do not need to exist anymore, as
  `.reuse/dep5` is used instead
- `reuse lint` makes it easy to ensure that copyright information of
  files like binary assets / images is always accurate and up to date

To add copyright information of files that didn't have it I looked up
who committed what and when, for each file. As yuzu contributors do not
have to sign a CLA or similar I couldn't assume that copyright ownership
was of the "yuzu Emulator Project", so I used the name and/or email of
the commit author instead.

[REUSE]: https://reuse.software

Follow-up to 01cf05bc75

externals/find-modules/FindCatch2.cmake

@@ -1,3 +1,5 @@
+# SPDX-FileCopyrightText: 2020 yuzu Emulator Project
+# SPDX-License-Identifier: GPL-2.0-or-later
 
 find_package(PkgConfig QUIET)
 pkg_check_modules(PC_Catch2 QUIET Catch2)

externals/find-modules/FindFFmpeg.cmake

@@ -1,9 +1,9 @@
+# SPDX-FileCopyrightText: 2019 Citra Emulator Project
+# SPDX-License-Identifier: GPL-2.0-or-later
+
 # FindFFmpeg
 # ----------
 #
-# Copyright 2019 Citra Emulator Project
-# Licensed under GPLv2 or any later version
-#
 # Find the native FFmpeg includes and libraries
 #
 # This module defines the following variables:

externals/find-modules/FindLibUSB.cmake

@@ -1,11 +1,12 @@
+# SPDX-FileCopyrightText: 2009 Michal Cihar <michal@cihar.com>
+# SPDX-License-Identifier: GPL-2.0-or-later
+
 # - Find libusb-1.0 library
 # This module defines
 #  LIBUSB_INCLUDE_DIR, where to find bluetooth.h
 #  LIBUSB_LIBRARIES, the libraries needed to use libusb-1.0.
 #  LIBUSB_FOUND, If false, do not try to use libusb-1.0.
 #
-# Copyright (c) 2009, Michal Cihar, <michal@cihar.com>
-#
 # vim: expandtab sw=4 ts=4 sts=4:
 
 if(ANDROID)

externals/find-modules/Findfmt.cmake

@@ -1,3 +1,5 @@
+# SPDX-FileCopyrightText: 2020 yuzu Emulator Project
+# SPDX-License-Identifier: GPL-2.0-or-later
 
 find_package(PkgConfig QUIET)
 pkg_check_modules(PC_fmt QUIET fmt)

externals/find-modules/Findlz4.cmake

@@ -1,3 +1,5 @@
+# SPDX-FileCopyrightText: 2020 yuzu Emulator Project
+# SPDX-License-Identifier: GPL-2.0-or-later
 
 find_package(PkgConfig QUIET)
 pkg_check_modules(PC_lz4 QUIET lz4)

externals/find-modules/Findnlohmann_json.cmake

@@ -1,3 +1,5 @@
+# SPDX-FileCopyrightText: 2020 yuzu Emulator Project
+# SPDX-License-Identifier: GPL-2.0-or-later
 
 find_package(PkgConfig QUIET)
 pkg_check_modules(PC_nlohmann_json QUIET nlohmann_json)

externals/find-modules/Findopus.cmake

@@ -1,3 +1,5 @@
+# SPDX-FileCopyrightText: 2020 yuzu Emulator Project
+# SPDX-License-Identifier: GPL-2.0-or-later
 
 find_package(PkgConfig QUIET)
 pkg_check_modules(PC_opus QUIET opus)

externals/find-modules/Findzstd.cmake

@@ -1,3 +1,5 @@
+# SPDX-FileCopyrightText: 2020 yuzu Emulator Project
+# SPDX-License-Identifier: GPL-2.0-or-later
 
 find_package(PkgConfig QUIET)
 pkg_check_modules(PC_zstd QUIET libzstd)