Fixed namespace code generation for C++

The code generator was assuming all declarations for the current
file sit in the same namepace. Now uses the "on demand" namespace
switching we had for the forward declarations.

Also fixed a bug related to namespace lookup.

Change-Id: Ib54a3efbc752cbb9590302fa0707c0c73448db3d
Tested: on Linux.

tests/namespace_test/NamespaceA/TableInC.py

@@ -0,0 +1,39 @@
+# automatically generated, do not modify
+
+# namespace: NamespaceA
+
+import flatbuffers
+
+class TableInC(object):
+    __slots__ = ['_tab']
+
+    # TableInC
+    def Init(self, buf, pos):
+        self._tab = flatbuffers.table.Table(buf, pos)
+
+    # TableInC
+    def ReferToA1(self):
+        o = flatbuffers.number_types.UOffsetTFlags.py_type(self._tab.Offset(4))
+        if o != 0:
+            x = self._tab.Indirect(o + self._tab.Pos)
+            from .TableInFirstNS import TableInFirstNS
+            obj = TableInFirstNS()
+            obj.Init(self._tab.Bytes, x)
+            return obj
+        return None
+
+    # TableInC
+    def ReferToA2(self):
+        o = flatbuffers.number_types.UOffsetTFlags.py_type(self._tab.Offset(6))
+        if o != 0:
+            x = self._tab.Indirect(o + self._tab.Pos)
+            from .SecondTableInA import SecondTableInA
+            obj = SecondTableInA()
+            obj.Init(self._tab.Bytes, x)
+            return obj
+        return None
+
+def TableInCStart(builder): builder.StartObject(2)
+def TableInCAddReferToA1(builder, referToA1): builder.PrependUOffsetTRelativeSlot(0, flatbuffers.number_types.UOffsetTFlags.py_type(referToA1), 0)
+def TableInCAddReferToA2(builder, referToA2): builder.PrependUOffsetTRelativeSlot(1, flatbuffers.number_types.UOffsetTFlags.py_type(referToA2), 0)
+def TableInCEnd(builder): return builder.EndObject()