Support targeting by package names, not just module names

[pschanely]

Packages are conspicuously missing from the set of things CrossHair can target. (though targeting by module is supported; see modules-vs-packages)

I think we should be able to use pkgutil.walk_packages to do a recursive package inspection.

A work-around until then: target by directory instead, e.g. crosshair check foo/bar/ instead of crosshair check foo.bar.