Reflect nested config files when walking directories

When a repository uses a mix of different licenses for different parts/directories (BUSL or MPL), then it is currently necessary to run `copywrite` in those nested directories in order for it to notice those nested directories.

For example, Terraform uses the following script to work around this limitation:

https://github.com/hashicorp/terraform/blob/2a5ff48e3d0342c1978adf97c63414fa06c8b9ed/scripts/copyright.sh#L12-L17

```sh
directories=$(find . -type f -name '.copywrite.hcl' -execdir pwd \;)

for dir in $directories; do
cd $dir && go run github.com/hashicorp/copywrite headers
done
```

It would be great if this could be built into the tool, so we would not have to maintain this shell wrapper.