[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/j6libc/aux.h

@@ -0,0 +1,57 @@
+#pragma once
+/* Auxiliary PDCLib code <aux.h>
+
+   This file is part of the Public Domain C Library (PDCLib).
+   Permission is granted to use, modify, and / or redistribute at will.
+*/
+
+/* -------------------------------------------------------------------------- */
+/* You should not have to edit anything in this file; if you DO have to, it   */
+/* would be considered a bug / missing feature: notify the author(s).         */
+/* -------------------------------------------------------------------------- */
+
+/* -------------------------------------------------------------------------- */
+/* Standard Version                                                           */
+/* -------------------------------------------------------------------------- */
+
+/* Many a compiler gets this wrong, so you might have to hardcode it instead. */
+
+#if __STDC__ != 1
+#error Compiler does not define _ _STDC_ _ to 1 (not standard-compliant)!
+#endif
+
+#ifdef __cplusplus
+#define restrict __restrict__
+#endif
+
+#ifndef __STDC_HOSTED__
+#error Compiler does not define _ _STDC_HOSTED_ _ (not standard-compliant)!
+#elif __STDC_HOSTED__ == 0
+#define _PDCLIB_HOSTED 0
+#elif __STDC_HOSTED__ == 1
+#define _PDCLIB_HOSTED 1
+#else
+#error Compiler does not define _ _STDC_HOSTED_ _ to 0 or 1 (not standard-compliant)!
+#endif
+
+/* -------------------------------------------------------------------------- */
+/* Helper macros:                                                             */
+/* _PDCLIB_cc( x, y ) concatenates two preprocessor tokens without extending  */
+/* _PDCLIB_concat( x, y ) concatenates two preprocessor tokens with extending */
+/* _PDCLIB_static_assert( e, m ) does a compile-time assertion of expression  */
+/*                               e, with m as the failure message.            */
+/* _PDCLIB_TYPE_SIGNED( type ) resolves to true if type is signed.            */
+/* _PDCLIB_TWOS_COMPLEMENT( type ) resolves to true if two's complement is    */
+/*                                 used for type.                             */
+/* -------------------------------------------------------------------------- */
+
+#define _PDCLIB_cc( x, y )     x ## y
+#define _PDCLIB_concat( x, y ) _PDCLIB_cc( x, y )
+
+#define _PDCLIB_static_assert( e, m ) enum { _PDCLIB_concat( _PDCLIB_assert_, __LINE__ ) = 1 / ( !!(e) ) }
+
+#define _PDCLIB_TYPE_SIGNED( type ) (((type) -1) < 0)
+#define _PDCLIB_TWOS_COMPLEMENT( type ) ((type) ~ (type) 0 < 0 )
+
+#define _PDCLIB_symbol2value( x ) #x
+#define _PDCLIB_symbol2string( x ) _PDCLIB_symbol2value( x )