ensure _dafny gets packaged, fix version number

When using the "flat layout", setuptools' auto-discovery only considers
a single top-level module.
See <https://setuptools.pypa.io/en/stable/userguide/package_discovery.html#flat-layout>.

Source/DafnyRuntime/DafnyRuntimePython/pyproject.toml

@@ -1,10 +1,6 @@
-[build-system]
-requires = ["setuptools>=61.0"]
-build-backend = "setuptools.build_meta"
-
 [project]
 name = "DafnyRuntimePython"
-version = "4.7.0"
+version = "4.8.1"
 authors = [
   { name = "The Dafny core team", email = "[email protected]" },
 ]
@@ -19,3 +15,10 @@ classifiers = [
 [project.urls]
 Homepage = "https://github.com/dafny-lang/dafny"
 Issues = "https://github.com/dafny-lang/dafny/issues"
+
+[build-system]
+requires = ["setuptools>=61.0"]
+build-backend = "setuptools.build_meta"
+
+[tool.setuptools]
+packages = ["_dafny", "System_"]