[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/float.h

@@ -0,0 +1,71 @@
+#pragma once
+/* Characteristics of floating types <float.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/config.h"
+
+#define FLT_ROUNDS      _PDCLIB_FLT_ROUNDS
+#define FLT_EVAL_METHOD _PDCLIB_FLT_EVAL_METHOD
+#define DECIMAL_DIG     _PDCLIB_DECIMAL_DIG
+
+   /* Radix of exponent representation */
+#define FLT_RADIX      __FLT_RADIX__
+   /* Number of base-FLT_RADIX digits in the significand of a float */
+#define FLT_MANT_DIG   __FLT_MANT_DIG__
+   /* Number of decimal digits of precision in a float */
+#define FLT_DIG        __FLT_DIG__
+   /* Difference between 1.0 and the minimum float greater than 1.0 */
+#define FLT_EPSILON    __FLT_EPSILON__
+   /* Minimum int x such that FLT_RADIX**(x-1) is a normalised float */
+#define FLT_MIN_EXP    __FLT_MIN_EXP__
+   /* Minimum normalised float */
+#define FLT_MIN        __FLT_MIN__
+   /* Minimum int x such that 10**x is a normalised float */
+#define FLT_MIN_10_EXP __FLT_MIN_10_EXP__
+   /* Maximum int x such that FLT_RADIX**(x-1) is a representable float */
+#define FLT_MAX_EXP    __FLT_MAX_EXP__
+   /* Maximum float */
+#define FLT_MAX        __FLT_MAX__
+   /* Maximum int x such that 10**x is a representable float */
+#define FLT_MAX_10_EXP __FLT_MAX_10_EXP__
+
+   /* Number of base-FLT_RADIX digits in the significand of a double */
+#define DBL_MANT_DIG   __DBL_MANT_DIG__
+   /* Number of decimal digits of precision in a double */
+#define DBL_DIG        __DBL_DIG__
+   /* Difference between 1.0 and the minimum double greater than 1.0 */
+#define DBL_EPSILON    __DBL_EPSILON__
+   /* Minimum int x such that FLT_RADIX**(x-1) is a normalised double */
+#define DBL_MIN_EXP    __DBL_MIN_EXP__
+   /* Minimum normalised double */
+#define DBL_MIN        __DBL_MIN__
+   /* Minimum int x such that 10**x is a normalised double */
+#define DBL_MIN_10_EXP __DBL_MIN_10_EXP__
+   /* Maximum int x such that FLT_RADIX**(x-1) is a representable double */
+#define DBL_MAX_EXP    __DBL_MAX_EXP__
+   /* Maximum double */
+#define DBL_MAX        __DBL_MAX__
+   /* Maximum int x such that 10**x is a representable double */
+#define DBL_MAX_10_EXP __DBL_MAX_10_EXP__
+
+   /* Number of base-FLT_RADIX digits in the significand of a long double */
+#define LDBL_MANT_DIG   __LDBL_MANT_DIG__
+   /* Number of decimal digits of precision in a long double */
+#define LDBL_DIG        __LDBL_DIG__
+   /* Difference between 1.0 and the minimum long double greater than 1.0 */
+#define LDBL_EPSILON    __LDBL_EPSILON__
+   /* Minimum int x such that FLT_RADIX**(x-1) is a normalised long double */
+#define LDBL_MIN_EXP    __LDBL_MIN_EXP__
+   /* Minimum normalised long double */
+#define LDBL_MIN        __LDBL_MIN__
+   /* Minimum int x such that 10**x is a normalised long double */
+#define LDBL_MIN_10_EXP __LDBL_MIN_10_EXP__
+   /* Maximum int x such that FLT_RADIX**(x-1) is a representable long double */
+#define LDBL_MAX_EXP    __LDBL_MAX_EXP__
+   /* Maximum long double */
+#define LDBL_MAX        __LDBL_MAX__
+   /* Maximum int x such that 10**x is a representable long double */
+#define LDBL_MAX_10_EXP __LDBL_MAX_10_EXP__