[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

scripts/bonnibel/__init__.py

@@ -1,3 +1,5 @@
+from os.path import join
+
 class BonnibelError(Exception):
     def __init__(self, message):
         self.message = message
@@ -6,3 +8,18 @@ def load_config(filename):
     from yaml import safe_load
     with open(filename, 'r') as infile:
         return safe_load(infile.read())
+
+def mod_rel(path):
+    return join("${module_dir}", path)
+
+def target_rel(path, module=None):
+    if module:
+        return join("${target_dir}", module + ".dir", path)
+    else:
+        return join("${target_dir}", path)
+
+def include_rel(path, module=None):
+    if module:
+        return join("${build_root}", "include", module, path)
+    else:
+        return join("${build_root}", "include", path)