Allow unions of table types with the same name but from different namespaces.

Also fixed most codegenerators using the wrong namespace when multiple
namespace were used in a file, with some files not being generated.

Change-Id: Ib42969221239d7244e431cbd667ef69200fc415f
Tested: on Linux.
Bug: 29338474

tests/MyGame/Example2/Monster.py

@@ -0,0 +1,15 @@
+# automatically generated by the FlatBuffers compiler, do not modify
+
+# namespace: Example2
+
+import flatbuffers
+
+class Monster(object):
+    __slots__ = ['_tab']
+
+    # Monster
+    def Init(self, buf, pos):
+        self._tab = flatbuffers.table.Table(buf, pos)
+
+def MonsterStart(builder): builder.StartObject(0)
+def MonsterEnd(builder): return builder.EndObject()