test(model_splitter): fix the scikit-learn requires_pkg marker

requires_pkg() resolves a distribution name and imports the matching module, and
scikit-learn matches neither name on its own: the distribution is scikit-learn and
the module is sklearn. test_save_load_node_mapping_structured was therefore skipped
everywhere, including the nightly optional dependency build, and had never run. Map
the distribution to its module so the test runs.

Author: jdhughes-dev

autotest/test_model_splitter.py

@@ -221,7 +221,7 @@ def test_metis_splitting_with_lak_sfr(function_tmpdir):
 @requires_exe("mf6")
 @requires_pkg("pymetis")
 @requires_pkg("h5py")
-@requires_pkg("sklearn")
+@requires_pkg("scikit-learn", name_map={"scikit-learn": "sklearn"})
 def test_save_load_node_mapping_structured(function_tmpdir):
     import pymetis