[scripts] Make j6libc.py into the codegen package

j6libc.py was initially made for libc to generate stdint.h, but it
gained a few things that aren't libc-specific. Move it to a package and
split the int-types-specific code into codegen.int_types.

src/libraries/libc_free/include/stdint.h.cog

@@ -12,7 +12,7 @@
 
 /**[[[cog code generation
 import cog
-from j6libc import definition, int_widths, int_mods
+from codegen.int_types import definition, int_widths, int_mods
 
 for width in int_widths:
     definition("typedef", f"__INT{width}_TYPE__",  f"int{width}_t;")