revamp cppcheck parser

Set the severity of Issue instances based on that from cppcheck

Treat <error> elements with no location as Failure instances, and add
a selftest for this:
  tests/parsers/example-output/cppcheck-xml-v2/example-002.xml

Fix a missing import of Notes that would affect any <error> for which
msg != verbose (though the only ones I've seen in the wild are of the form
of example-002.xml)