python,scons: Move grammar.py and code_formatter.py into build_tools.

These are only used in a build, and so don't need to be built into gem5.
grammar.py is used by slicc and the fast model project file parser, and
code_formatter.py is only used by SConscripts.

Change-Id: Id43e62459d69f07fdb2ed125548a83e38bbb7590
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49396
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Maintainer: Gabe Black <gabe.black@gmail.com>

src/SConscript

@@ -68,7 +68,7 @@ Export('env')
 
 build_env = [(opt, env[opt]) for opt in export_vars]
 
-from m5.util import code_formatter
+from code_formatter import code_formatter
 
 def GdbXml(xml_id, symbol):
     cc, hh = env.Blob(symbol, xml_id)