The long-term goal is to deprecate imp.find_module() in favour of this
API, but it will take some time as some APIs explicitly return/use what
imp.find_module() returns.
and relies much more on meta path finders to abstract out various parts of
import.
As part of this the semantics for import_module tightened up and now follow
__import__ much more closely (biggest thing is that the 'package' argument must
now already be imported, else a SystemError is raised).