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.go

@@ -0,0 +1,18 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+package Example2
+
+import (
+	flatbuffers "github.com/google/flatbuffers/go"
+)
+type Monster struct {
+	_tab flatbuffers.Table
+}
+
+func (rcv *Monster) Init(buf []byte, i flatbuffers.UOffsetT) {
+	rcv._tab.Bytes = buf
+	rcv._tab.Pos = i
+}
+
+func MonsterStart(builder *flatbuffers.Builder) { builder.StartObject(0) }
+func MonsterEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT { return builder.EndObject() }