Gprclean doesn't support -vPx

[simonjwright]

The GPRBuild Companion Tools documentation, and the runtime help, both say that gprclean should accept -vPx (the runtime help doesn't say what 'x' should be!)

This doesn't work, because of this:

            elsif In_Package_Clean then
               Bad_Switch;

(looking at In_Package_Clean, it seems to mean "we are dealing with an actual project, not the default one" - in which case it would be nice to be able to see verbose reports of the processing).