discovery: handle deletion of 'wireproto' module
Mercurial commit b4d85bc122bd. deleted the wireproto module and its
contents are now in various other modules, so we can not even do
unchecked imports of it now.
#require test-repo pyflakes
Copied from Mercurial core (60ee2593a270)
$ cd "`dirname "$TESTDIR"`"
run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)
$ hg locate 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
> | xargs pyflakes 2>/dev/null