[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/util/util.module

@@ -2,8 +2,24 @@
 
 module("util",
     kind = "lib",
-    includes = [ "include" ],
     sources = [
         "bip_buffer.cpp",
         "spinlock.cpp",
+    ],
+    public_headers = [
+        "util/basic_types.h",
+        "util/bip_buffer.h",
+        "util/constexpr_hash.h",
+        "util/counted.h",
+        "util/deque.h",
+        "util/enum_bitfields.h",
+        "util/hash.h",
+        "util/linked_list.h",
+        "util/map.h",
+        "util/misc.h",
+        "util/no_construct.h",
+        "util/pointers.h",
+        "util/spinlock.h",
+        "util/util.h",
+        "util/vector.h",
     ])