Fixes lp:1560134

aec68a784 added module names to error messages,
however it caused a TypeError for relative imports
that do not specify a module such as:

   from . import x

This fixes the TypeError, and also adds the necessary
leading dots for relative import error messages.

Add tests for various types of relative imports.

Fixes lp:1578051

aec68a7 added module names to error messages, which included
a new class SubmoduleImportation to handle the special case
of submodule imports.  It correctly handled the case of
a submodule import occurring after the root module was imported,
but didnt handle the opposite case of the submodule import
occurring before the root module was imported.

In order to solve many corner cases related to imports,
more information is needed about each import.

This change creates two new classes:
- SubmoduleImportation
- ImportationFrom

And adds an optional parameter full_name to the super class
Importation.

Functionally, this change only improves existing error messages
to report the full imported name where previously an error
would include only the import alias.

As '*' does not appear in __future__.all_feature_names, this
also reports an error on : from __future__ import *

Also detect unused star imports.

An import in a class is a member of the class,
and may be used outside the class scope, so it
can not be marked as unused.

When an import binds to a name declared global by the "global"
statment, don't report it as unused.

Add declared globals to each scope in the scope stack to avoid
erroneous UndefinedName errors when they are loaded.