tools/licensegen: consider COPYING files

[tklauser]

Some projects use a COPYING file for their license. Consider these as
well. Currently, the only vendored dependency using such a file is
github.com/BurntSushi/toml:

$ diff -u LICENSE.all LICENSE.all.new
--- LICENSE.all	2021-03-08 11:12:05.281088204 +0100
+++ LICENSE.all.new	2021-03-08 11:11:38.757023948 +0100
@@ -2132,6 +2132,29 @@
    See the License for the specific language governing permissions and
    limitations under the License.

+Name: vendor/github.com/BurntSushi/toml/COPYING
+License: The MIT License (MIT)
+
+Copyright (c) 2013 TOML authors
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.
+
 Name: vendor/github.com/Microsoft/go-winio/LICENSE
 License: The MIT License (MIT)

[rolinh]

Nice catch!