[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/project.py

@@ -15,7 +15,6 @@ class Project:
         import bonnibel
         from os.path import join
         from ninja.ninja_syntax import Writer
-        from . import load_config
         from .target import Target
 
         targets = set()
@@ -40,10 +39,31 @@ class Project:
             build.variable("version_sha", self.version.sha)
             build.newline()
 
+            build.variable("cogflags", [
+                "-I", "${source_root}/scripts",
+                "-D", "definitions_path=${source_root}/definitions",
+                ])
+            build.newline()
+
             for target in targets:
                 build.subninja(output / target.name / "target.ninja")
             build.newline()
 
+            for mod in modules.values():
+                build.subninja(output / f"headers.{mod.name}.ninja")
+            build.newline()
+
+            build.build(
+                rule = "touch",
+                outputs = "${build_root}/.all_headers",
+                implicit = [f"${{build_root}}/include/{m.name}/.headers.phony"
+                    for m in modules.values() if m.public_headers],
+                )
+            build.build(
+                rule = "phony",
+                outputs = ["all-headers"],
+                inputs = ["${build_root}/.all_headers"])
+
             debugroot = output / ".debug"
             debugroot.mkdir(exist_ok=True)
 
@@ -82,7 +102,6 @@ class Project:
                     })
 
                 add_fatroot(intermediary, entry)
-                return mod.location
 
             from .manifest import Manifest
             manifest = Manifest(manifest_file, modules)