Fix unexpected-AttributeError-when-importing

The repro is:
- import a.b, and in a.b do
- import a.b as ab

This will cause an attribute error, since a.b (the name) is successfully importable,
but a.b (the attribute) doesn't exist yet.

src/core/cfg.cpp

@@ -1700,8 +1700,10 @@ public:
                         l = r + 1;
                         continue;
                     }
-                    pushAssign(tmpname, new AST_Attribute(makeLoad(tmpname, node, true), AST_TYPE::Load,
-                                                          internString(a->name.s().substr(l, r - l))));
+                    auto attr = new AST_Attribute(makeLoad(tmpname, node, true), AST_TYPE::Load,
+                                                  internString(a->name.s().substr(l, r - l)));
+                    attr->lineno = import->lineno;
+                    pushAssign(tmpname, attr);
                     l = r + 1;
                 } while (l < a->name.s().size());
                 pushAssign(a->asname, makeLoad(tmpname, node, true));

test/tests/test_package/import_target.py

 print "running test_package.import_target"
+
+# Since we are currently importing test_package.import_target, this
+# import will succeed (return directly from sys.modules), even though
+# test_package will not have the 'import_target' attribute yet
+
+import test_package.import_target
+
+try:
+    print test_package.import_target
+    assert 0
+except AttributeError:
+    pass
+
+try:
+    print getattr(test_package, 'import_target')
+    assert 0
+except AttributeError:
+    pass
+
+# You can do 'import test_package.import_target', but adding an asname will cause an exception:
+try:
+    import test_package.import_target as i
+    assert 0
+    i
+except AttributeError:
+    pass