[ament_copyright] idea: include checking of package.xml and maybe LICENSE files

Currently, the "license" statement in the package.xml is not checked by ament_copyright, neither whether it is valid, nor whether it matches the statements in the code (the code-statements legally supersede the package.xml, but conflicts are never good). Also, some packages have "LICENSE" files in the root, and again, it would be useful to check those for consistency.

How about adding this? If you are in favor that this makes, would you add it to ament_copyright, or create a new linter?