package.xml linter that can be a little more opinionated than XSD allows

Define and enforce a style guide via automatic linter for `package.xml` files

Immediate ones that come to mind that I am clocking manually in reviews:
1. Keep dependency types together in blocks separated by newlines
2. Keep dependencies within each group in alphabetical order

Maybe this doesn't belong at `ament` level which is theoretically "ros-agnostic", but not sure where else to open a tracking issue.