Refactor Python "gdb" module into a proper Python package, by introducing

Refactor Python "gdb" module into a proper Python package, by introducing
a new "_gdb" module for code implemented in C, and using reload/__import__
instead of exec.

gdb/

	* python/lib/gdb/__init__.py: Import * from _gdb.
	(GdbOutputFile, sys.stdout, GdbOutputErrorFile, sys.stderr,
	prompt_hook, sys.argv): Moved from finish_python_initialization.
	(pretty_printers, PYTHONDIR): Moved from _initialize_python.
	(packages, auto_load_packages): New list and function replacing
	module_dict and auto-loading code, using __file__ instead of
	gdb.PYTHONDIR and reload/__import__ instead of exec.
	(GdbSetPythonDirectory): Replacing function of the same name
	from finish_python_initialization, using reload/__import__ instead
	of exec, as well as call auto_load_packages.
	* python/py-prettyprint.c (find_pretty_printer_from_gdb): Check
	gdb_python_module and not gdb_module.
	* python/python-internal.h (gdb_python_module): Declare.
	* python/python.c (gdb_python_module): New global.
	(before_prompt_hook): Check gdb_python_module and not gdb_module.
	(_initialize_python): Rename gdb module to _gdb.
	Move gdb.PYTHONDIR and gdb.pretty_printer to lib/gdb/__init__.py.
	(finish_python_initialization): Move Python code to
	lib/gdb/__init__.py; instead, set up sys.path and import gdb into
	__main__.

gdb/testsuite/

	* gdb.python/python.exp (Test stderr location): Update module
	location of GDB-specific sys.stderr.
	(Test stdout location): Ditto for sys.stdout.