refactor the crappy sbom generator as an aspect

diff --git a/tools/sbom/BUILD b/tools/sbom/BUILD
index 71a4d5e..ff86ba5 100644
--- a/tools/sbom/BUILD
+++ b/tools/sbom/BUILD

@@ -1,5 +1,19 @@
-# SBOM generator
-load("@rules_python//python:defs.bzl", "py_binary")
+# BUILD file defining @rules_license/tools/sbom
+#
+# Copyright 2020 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Sample SBOM generator."""
 
 package(
     default_package_metadata = ["//:license"],
@@ -7,10 +21,19 @@
 )
 
 filegroup(
-    name = "srcs",
+    name = "standard_package",
     srcs = glob(["**"]),
 )
 
+# Do not create a bzl_library(). That would create a dependency loop back
+# to bazel-skylib. We export the .bzl files to the documentation maker.
+exports_files(
+    glob([
+        "*.bzl",
+    ]),
+    visibility = ["//doc_build:__pkg__"],
+)
+
 py_binary(
     name = "write_sbom_internal",
     srcs = ["write_sbom.py"],

diff --git a/tools/sbom/sbom.bzl b/tools/sbom/sbom.bzl
index 48d9eac..ae95c2d 100644
--- a/tools/sbom/sbom.bzl
+++ b/tools/sbom/sbom.bzl

@@ -13,7 +13,13 @@
 # limitations under the License.
 """Generate an SBOM for a target."""
 
-load("//rules_gathering:gather_packages.bzl", "gather_package_info", "packages_used")
+load(
+    "//rules_gathering:gather_packages.bzl",
+    "gather_package_info",
+    "packages_used",
+    "write_packages_info",
+    "TransitivePackageInfo"
+)
 
 def _spdx_common(ctx, target, spdx_output, _gen_spdx_tool):
     # Gather all licenses and write information to one place
@@ -49,11 +55,12 @@
     return [
         DefaultInfo(files = depset(outputs)),
         OutputGroupInfo(
-            spdx = depset(outputs),
+            sbom_spdx = depset(outputs),
         ),
     ]
 
 def _sbom_impl(ctx):
+    print("TOAST1")
     _create_sbom(ctx, ctx.file.packages_used, ctx.outputs.out, ctx.executable._sbom_generator)
 
 _sbom = rule(
@@ -111,33 +118,29 @@
         maven_install = maven_install,
     )
 
-def _gen_spdx_impl(target, ctx):
-    """
+def _gen_sbom_spdx_impl(target, ctx):
+    print("TOAST")
+    info_aspect_output = ctx.actions.declare_file("%s_info.json" % ctx.label.name)
+    # traverse output from aspect and assemble it for writing...
+    write_packages_info(
+        ctx,
+        top_level_target = target,
+        transitive_package_info = target[TransitivePackageInfo],
+        output = info_aspect_output,
+    )
     spdx_output = ctx.actions.declare_file("%s.spdx.json" % ctx.label.name)
-
-    name = "%s_info.json" % ctx.label.name
-    aspect_output = ctx.actions.declare_file(name)
-
-    # ... possibly traverse output from aspect and assemble it for writing...
-    info = target[TransitiveLicensesInfo]
-
-    # If the result doesn't contain licenses, we simply return the provider
-    #if not hasattr(info, "target_under_license"):
-    #    return [OutputGroupInfo()]
-
-    content = "[\n%s\n]\n" % ",\n".join(info_to_json(info))
-    """
-
-    #return _spdx_common(ctx, target, spdx_output, ctx.executable._gen_spdx)
-    return _create_sbom(ctx, ctx.file.packages_used, ctx.output.out, ctx.executable._sbom_generator)
+    print("WRITE TO", spdx_output.path)
+    return _create_sbom(ctx, info_aspect_output, spdx_output, ctx.executable._gen_spdx)
 
 
 gen_sbom_spdx = aspect(
-    implementation = _gen_spdx_impl,
+    doc = """Generates an SPDX sbom for a target.""",
+    implementation = _gen_sbom_spdx_impl,
     requires = [gather_package_info],
     attrs = {
         "_gen_spdx": attr.label(
             default = Label("//tools/sbom:write_sbom_internal"),
+            allow_files = True,
             executable = True,
             cfg = "exec",
         ),

diff --git a/tools/sbom/write_sbom.py b/tools/sbom/write_sbom.py
index 1982124..3373379 100644
--- a/tools/sbom/write_sbom.py
+++ b/tools/sbom/write_sbom.py

@@ -243,7 +243,7 @@
       # Useful for debugging
 
 
-  maven_packages = None
+  maven_packages = {}
   if opts.maven_install:
     with open(opts.maven_install, "rt", encoding="utf-8") as inp:
       maven_install = json.loads(inp.read())