dx2cg: Allow usage as Python module

e.g.
from dx2cg.swapper import process_shader
cg_shader = process_shader(text)

dx2cg/main.py

@@ -3,7 +3,7 @@
 
 import os
 import sys
-from swapper import process
+from .swapper import process
 
 def process_file(filename, suffix):
     dot = filename.rfind(".")

dx2cg/swapper.py

@@ -5,7 +5,7 @@
 
 import re
 import sys
-from disassembler import disassemble
+from .disassembler import disassemble
 
 tabs = 3
 def indent(block):