Fix double import of _lldb module.

Fix llvm.org/pr39054:
- Register _lldb as a built-in module during initialization of script interpreter,
- Reverse the order of imports in __init__.py: first try to import by absolute name, which will find the built-in module in the context of lldb (and other hosts that embed liblldb), then try relative import, in case the module is being imported from Python interpreter.

This works for SWIG>=3.0.11; before that, SWIG did not support custom module import code.

Differential revision: https://reviews.llvm.org/D52404

llvm-svn: 344474

lldb/source/Plugins/ScriptInterpreter/Python/ScriptInterpreterPython.cpp

@@ -132,6 +132,9 @@ public:
 
     InitializePythonHome();
 
+    // Register _lldb as a built-in module.
+    PyImport_AppendInittab("_lldb", g_swig_init_callback);
+
 // Python < 3.2 and Python >= 3.2 reversed the ordering requirements for
 // calling `Py_Initialize` and `PyEval_InitThreads`.  < 3.2 requires that you
 // call `PyEval_InitThreads` first, and >= 3.2 requires that you call it last.