[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/include/errno.h.cog

@@ -12,9 +12,9 @@
 
 
 /**[[[cog code generation
-from j6libc import arch_includes
+from codegen import arch_includes
 
-arch_includes("errno.h")
+arch_includes("errno.h", root="__j6libc")
 
 ]]]*/
 /*[[[end]]]*/

src/libraries/libc/include/inttypes.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("#define", f"PRId{width}", f"__INT{width}_FMTd__")

src/libraries/libc/include/stdatomic.h.cog

@@ -18,7 +18,7 @@ extern "C" {
 #endif
 
 /**[[[cog code generation
-from j6libc import atomic_types
+from codegen.int_types import atomic_types
 
 deftypes = ["BOOL", "CHAR16", "CHAR32", "CHAR", "INT",
             "LLONG", "SHORT", "WCHAR_T", "POINTER"]