The `check-conventions` script inadvertently ignored checking the
license header in `CMakeLists.txt`. Change the script so that those are
checked. Also add the missing headers to the reported files.
Drop using the `revng-` prefix from all executables under
`libexec/revng/`. Now any executable found under there that is
executable and without extension will be considered a `revng`
subcommand, following the usual command-line rules.
Alessandro Di Federico