MNT(healpy,ducc): add external dependency explainer (#171)

Use the `external_dependency_explainer` helper in the `heracles.healpy`
and `heracles.ducc` module when optional external dependencies are
imported.

Closes: #170

heracles/ducc.py

@@ -24,10 +24,12 @@
 
 from typing import TYPE_CHECKING
 
-import ducc0
 import numpy as np
 
-from heracles.core import update_metadata
+from heracles.core import external_dependency_explainer, update_metadata
+
+with external_dependency_explainer:
+    import ducc0
 
 if TYPE_CHECKING:
     from collections.abc import Mapping

heracles/healpy.py

@@ -25,11 +25,13 @@
 from functools import cached_property, wraps
 from typing import TYPE_CHECKING
 
-import healpy as hp
 import numpy as np
 from numba import njit
 
-from heracles.core import update_metadata
+from heracles.core import external_dependency_explainer, update_metadata
+
+with external_dependency_explainer:
+    import healpy as hp
 
 if TYPE_CHECKING:
     from collections.abc import Mapping