relicense: add spack license command

- `spack license list-files`: list all files that should have license headers
- `spack license list-lgpl`:  list files still under LGPL-2.1
- `spack license verify`:     check that license headers are correct

- Added `spack license verify` to style tests

share/spack/qa/run-flake8-tests

@@ -17,4 +17,8 @@
 . "$(dirname $0)/setup.sh"
 check_dependencies flake8
 
+# verify that the code style is correct
 spack flake8
+
+# verify that the license headers are present
+spack license verify