Yields ModuleInfo for all modules recursively
on path, or, if path is None, all accessible modules.
'path' should be either None or a list of paths to look for
modules in.
'prefix' is a string to output on the front of every module name
on output.
Note that this function must import all *packages* (NOT all
modules!) on the given path, in order to access the __path__
attribute to find submodules.
'onerror' is a function which gets called with one argument (the
name of the package which was being imported) if any exception
occurs while trying to import a package. If no onerror function is
supplied, ImportErrors are caught and ignored, while all other
exceptions are propagated, terminating the search.
Examples:
# list all modules python can access
walk_packages()
# list all submodules of ctypes
walk_packages(ctypes.__path__, ctypes.__name__+'.')