fix: don't import from gt4py directly (and fix UW import paths)

[romanc]

This PR changes the import paths to import "gt4py things" through the interface that we re-expose in NDSL, see NOAA-GFDL/NDSL#126 for context.

The import path ndsl.dsl.gtscript doesn't exist and thus crashes any attempt of currently running translate, e.g.

(...)/pyMoist/tests/scripts (dsl/develop) $ ./run_tests.sh ../../test_data/from_charles/ dace:cpu GFDL_1M_Finalize
2026-02-11 14:19:33|INFO|rank 0|ndsl.logging:Constant selected: ConstantVersions.UFS
ImportError while loading conftest '(...)/pyMoist/tests/conftest.py'.
../conftest.py:1: in <module>
    import savepoint
../savepoint/__init__.py:22: in <module>
    from .translate_compute_uwshcu import TranslateComputeUwshcuInv
../savepoint/translate_compute_uwshcu.py:9: in <module>
    from pyMoist.UW.compute_uwshcu import ComputeUwshcuInv
../../pyMoist/UW/compute_uwshcu.py:6: in <module>
    from ndsl.dsl.gtscript import (
E   ModuleNotFoundError: No module named 'ndsl.dsl.gtscript'

since UW tests are auto-detected and those pull in UW source code (which has the non-existing import paths).

[romanc]

closing as we are in the process of transitioning to current GEOS v11. until then, no further work is planned on dsl/develop.