[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

configs/rules.ninja

@@ -1,6 +1,6 @@
 # This file is automatically generated by bonnibel
 
-rule compile_c
+rule compile.c
   command = $cc -MMD -MF $out.d $cflags $ccflags -o $out -c $in
   description = Compiling $name
   depfile = $out.d
@@ -15,7 +15,7 @@ rule dump_c_run
       $out; chmod a+x $out
   description = Dumping C arguments for $target
 
-rule compile_cxx
+rule compile.cpp
   command = $cxx -MMD -MF $out.d $cxxflags $ccflags -o $out -c $in
   description = Compiling $name
   depfile = $out.d
@@ -30,14 +30,14 @@ rule dump_cpp_run
       >> $out; chmod a+x $out
   description = Dumping C++ arguments for $target
 
-rule compile_asm
+rule compile.s
   command = $nasm -o $out -felf64 -MD $out.d $asflags $in
   description = Assembling $name
   depfile = $out.d
   deps = gcc
 
-rule parse_cog
-  command = $cog -o $out -d -D target=$target $cogflags $in
+rule parse.cog
+  command = cog -o $out -d -D target=$target $cogflags $in
   description = Parsing $name
 
 rule exe