import-checker: resolve relative imports

"from . import X" will produce an ImportFrom ast node with .module =
None. This resulted in a run-time error from attempting to concatenate
None with a str.

Another problem with relative imports is that the prefix may be dynamic
based on the "level" attribute of the import. e.g. "from ." has level 1
and "from .." has level 2.

We teach the "fromlocal" function how to cope with relative imports.
Where appropriate, the consumer passes in the level so relative module
names may be resolved properly.

contrib/import-checker.py

@@ -78,13 +78,26 b' def fromlocalfunc(modulename, localmods)'
     >>> # unknown = maybe standard library
     >>> fromlocal('os')
     False
+    >>> fromlocal(None, 1)
+    ('foo', 'foo.__init__', True)
+    >>> fromlocal2 = fromlocalfunc('foo.xxx.yyy', localmods)
+    >>> fromlocal2(None, 2)
+    ('foo', 'foo.__init__', True)
     """
     prefix = '.'.join(modulename.split('.')[:-1])
     if prefix:
         prefix += '.'
-    def fromlocal(name):
-        # check relative name at first
-        for n in prefix + name, name:
+    def fromlocal(name, level=0):
+        # name is None when relative imports are used.
+        if name is None:
+            # If relative imports are used, level must not be absolute.
+            assert level > 0
+            candidates = ['.'.join(modulename.split('.')[:-level])]
+        else:
+            # Check relative name first.
+            candidates = [prefix + name, name]
+
+        for n in candidates:
             if n in localmods:
                 return (n, n, False)
             dottedpath = n + '.__init__'
@@ -239,7 +252,7 b' def imported_modules(source, modulename,'
                     continue
                 yield found[1]
         elif isinstance(node, ast.ImportFrom):
-            found = fromlocal(node.module)
+            found = fromlocal(node.module, node.level)
             if not found:
                 # this should import standard library
                 continue