feat(gazelle_python_manifest): provide a target with the given name (#787)

gazelle/manifest/defs.bzl

diff --git a/gazelle/manifest/defs.bzl b/gazelle/manifest/defs.bzl
index a675e25..bbcda72 100644
--- a/gazelle/manifest/defs.bzl
+++ b/gazelle/manifest/defs.bzl
@@ -89,3 +89,10 @@
         },
         visibility = ["//visibility:private"],
     )
+
+    native.filegroup(
+        name = name,
+        srcs = [manifest],
+        tags = ["manual"],
+        visibility = ["//visibility:public"],
+    )