Bring back COPYING

Some things want this file to exist (e.g., the Fedora RPM spec, GitHub;
see osandov#221). Let's do what the Linux kernel does and provide a summary of
the licensing situation in COPYING. The REUSE FAQ [1] doesn't discourage
this.

1: https://reuse.software/faq/#tradition

Signed-off-by: Omar Sandoval <osandov@osandov.com>

COPYING

@@ -0,0 +1,6 @@
+drgn is provided under:
+
+	SPDX-License-Identifier: LGPL-2.1-or-later
+
+Some source files are provided under different licenses as noted in each file.
+See the LICENSES directory for the full list of licenses used.

MANIFEST.in

@@ -5,4 +5,4 @@ include tests/linux_kernel/kmod/Makefile tests/linux_kernel/kmod/drgn_test.c
 recursive-include tools *.py
 recursive-include vmtest *.c *.py *.rst
 recursive-include LICENSES
-include .flake8 CONTRIBUTING.rst pytest.ini util.py
+include .flake8 CONTRIBUTING.rst COPYING pytest.ini util.py