Correct split_source test

As noted by @layday, `ns["package_dir"].keys() == {"", "module_b"}`
should always be true, so we don't have to test for module_a in
`ns["package_dir"]`.

tests/masonry/builders/test_sdist.py

@@ -644,4 +644,4 @@ def test_split_source():
  setup_ast.body = [n for n in setup_ast.body if isinstance(n, ast.Assign)]
  ns = {}
  exec(compile(setup_ast, filename="setup.py", mode="exec"), ns)
- assert "module_a" in ns["package_dir"] or "module_b" in ns["package_dir"]
+ assert ns["package_dir"] == {"": "lib_a", "module_b": "lib_b/module_b"}