Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tools/buck/prelude/python/source_db.bzl

@@ -0,0 +1,60 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+#
+# This source code is licensed under both the MIT license found in the
+# LICENSE-MIT file in the root directory of this source tree and the Apache
+# License, Version 2.0 found in the LICENSE-APACHE file in the root directory
+# of this source tree.
+
+load(
+    ":manifest.bzl",
+    "ManifestInfo",  # @unused Used as a type
+)
+load(":python.bzl", "PythonLibraryManifestsTSet")
+load(":toolchain.bzl", "PythonToolchainInfo")
+
+def create_source_db(
+        ctx: "context",
+        srcs: [ManifestInfo.type, None],
+        python_deps: ["PythonLibraryInfo"]) -> DefaultInfo.type:
+    output = ctx.actions.declare_output("db.json")
+    artifacts = []
+
+    python_toolchain = ctx.attrs._python_toolchain[PythonToolchainInfo]
+    cmd = cmd_args(python_toolchain.make_source_db)
+    cmd.add(cmd_args(output.as_output(), format = "--output={}"))
+
+    # Pass manifests for rule's sources.
+    if srcs != None:
+        cmd.add(cmd_args(srcs.manifest, format = "--sources={}"))
+        artifacts.extend(srcs.artifacts)
+
+    # Pass manifests for transitive deps.
+    dep_manifests = ctx.actions.tset(PythonLibraryManifestsTSet, children = [d.manifests for d in python_deps])
+
+    dependencies = cmd_args(dep_manifests.project_as_args("source_type_manifests"), format = "--dependency={}")
+    dependencies_file = ctx.actions.write("source_db_dependencies", dependencies)
+    dependencies_file = cmd_args(dependencies_file, format = "@{}").hidden(dependencies)
+
+    cmd.add(dependencies_file)
+    artifacts.append(dep_manifests.project_as_args("source_type_artifacts"))
+
+    ctx.actions.run(cmd, category = "py_source_db")
+
+    return DefaultInfo(default_outputs = [output], other_outputs = artifacts)
+
+def create_source_db_no_deps(
+        ctx: "context",
+        srcs: [{str.type: "artifact"}, None]) -> DefaultInfo.type:
+    content = {} if srcs == None else srcs
+    output = ctx.actions.write_json("db_no_deps.json", content)
+    return DefaultInfo(default_outputs = [output], other_outputs = content.values())
+
+def create_source_db_no_deps_from_manifest(
+        ctx: "context",
+        srcs: ManifestInfo.type) -> DefaultInfo.type:
+    output = ctx.actions.declare_output("db_no_deps.json")
+    cmd = cmd_args(ctx.attrs._python_toolchain[PythonToolchainInfo].make_source_db_no_deps)
+    cmd.add(cmd_args(output.as_output(), format = "--output={}"))
+    cmd.add(srcs.manifest)
+    ctx.actions.run(cmd, category = "py_source_db")
+    return DefaultInfo(default_outputs = [output], other_outputs = srcs.artifacts)