bpo-37409: fix relative import with no parent (#14956)

Relative imports use resolve_name to get the absolute target name,
which first seeks the current module's absolute package name from the globals:
If __package__ (and __spec__.parent) are missing then
import uses __name__, truncating the last segment if
the module is a submodule rather than a package __init__.py
(which it guesses from whether __path__ is defined).

The __name__ attempt should fail if there is no parent package (top level modules),
if __name__ is '__main__' (-m entry points), or both (scripts).
That is, if both __name__ has no subcomponents and the module does not seem
to be a package __init__ module then import should fail.

Lib/test/test_import/__init__.py

@@ -772,6 +772,11 @@ class RelativeImportTests(unittest.TestCase):
         ns = dict(__package__=object())
         self.assertRaises(TypeError, check_relative)
 
+    def test_parentless_import_shadowed_by_global(self):
+        # Test as if this were done from the REPL where this error most commonly occurs (bpo-37409).
+        script_helper.assert_python_failure('-W', 'ignore', '-c',
+            "foo = 1; from . import foo")
+
     def test_absolute_import_without_future(self):
         # If explicit relative import syntax is used, then do not try
         # to perform an absolute import in the face of failure.