[build] Move headers out of target dirs

The great header shift: It didn't make sense to regenerate headers for
the same module for every target (boot/kernel/user) it appeared in. And
now that core headers are out of src/include, this was going to cause
problems for the new libc changes I've been working on. So I went back
to re-design how module headers work.

Pre-requisites:
- A module's public headers should all be available in one location, not
  tied to target.
- No accidental includes. Another module should not be able to include
  anything (creating an implicit dependency) from a module without
  declaring an explicit dependency.
- Exception to the previous: libc's headers should be available to all,
  at least for the freestanding headers.

New system:
- A new "public_headers" property of module declares all public headers
  that should be available to dependant modules
- All public headers (after possible processing) are installed relative
  to build/include/<module> with the same path as their source
- This also means no "include" dir in modules is necessary. If a header
  should be included as <j6/types.h> then its source should be
  src/libraries/j6/j6/types.h - this caused the most churn as all public
  header sources moved one directory up.
- The "includes" property of a module is local only to that module now,
  it does not create any implicit public interface

Other changes:
- The bonnibel concept of sources changed: instead of sources having
  actions, they themselves are an instance of a (sub)class of Source,
  which provides all the necessary information itself.
- Along with the above, rule names were standardized into <type>.<ext>,
  eg "compile.cpp" or "parse.cog"
- cog and cogflags variables moved from per-target scope to global scope
  in the build files.
- libc gained a more dynamic .module file

src/libraries/libc/limits.h

@@ -0,0 +1,36 @@
+#pragma once
+/* Sizes of integer types <limits.h>
+
+   This file is part of the Public Domain C Library (PDCLib).
+   Permission is granted to use, modify, and / or redistribute at will.
+*/
+
+#include "j6libc/int.h"
+
+/* TODO: Defined to 1 as multibyte characters are not supported yet. */
+#define MB_LEN_MAX 1
+
+#define SCHAR_MAX  __SCHAR_MAX__
+#define SCHAR_MIN  ((-SCHAR_MAX) - 1)
+#define UCHAR_MAX  (((unsigned char)SCHAR_MAX << 1) | 1)
+
+#define CHAR_BIT   __CHAR_BIT__
+#define CHAR_MAX   SCHAR_MAX
+#define CHAR_MIN   SCHAR_MIN
+
+#define SHRT_MAX   __SHRT_MAX__
+#define SHRT_MIN   ((-SHRT_MAX) - 1)
+#define USHRT_MAX  (((unsigned short)SHRT_MAX << 1) | 1)
+
+#define INT_MAX    __INT_MAX__
+#define INT_MIN    ((-INT_MAX) - 1)
+#define UINT_MAX   (((unsigned int)INT_MAX << 1) | 1)
+
+#define LONG_MAX   __LONG_MAX__
+#define LONG_MIN   ((-LONG_MAX) - 1)
+#define ULONG_MAX  (((unsigned long)LONG_MAX << 1) | 1)
+
+#define LLONG_MAX  __LONG_LONG_MAX__
+#define LLONG_MIN  ((-LLONG_MAX) - 1)
+#define ULLONG_MAX (((unsigned long long)LLONG_MAX << 1) | 1)
+