Fix pkg_resources->importlib performance regression (cctbx#658)

Regression observed in large experiment lists with scaling/profile models.

Only seems to affect Python 3.10, potentially stemming from:
    https://bugs.python.org/issue44246

Fixes cctbx#657

newsfragments/658.bugfix

@@ -0,0 +1 @@
+Fix performance regression loading large experiment lists containing profile/scaling models.

src/dxtbx/model/experiment_list.py

@@ -59,6 +59,14 @@ class InvalidExperimentListError(RuntimeError):
     """
 
 
+try:
+    scaling_model_entry_points = importlib.metadata.entry_points()[
+        "dxtbx.scaling_model_ext"
+    ]
+except KeyError:
+    scaling_model_entry_points = []
+
+
 class FormatChecker:
     """A helper class to speed up identifying the correct image format by first
     trying the last format that was used."""
@@ -576,7 +584,7 @@ def _lookup_model(self, name, experiment_dict):
     @staticmethod
     def _scaling_model_from_dict(obj):
         """Get the scaling model from a dictionary."""
-        for entry_point in importlib.metadata.entry_points()["dxtbx.scaling_model_ext"]:
+        for entry_point in scaling_model_entry_points:
             if entry_point.name == obj["__id__"]:
                 return entry_point.load().from_dict(obj)
 

src/dxtbx/model/profile.py

@@ -3,6 +3,11 @@
 import importlib.metadata
 import logging
 
+try:
+    profile_entry_points = importlib.metadata.entry_points()["dxtbx.profile_model"]
+except KeyError:
+    profile_entry_points = []
+
 
 class ProfileModelFactory:
     """
@@ -16,7 +21,7 @@ def from_dict(obj):
         """
         if obj is None:
             return None
-        for entry_point in importlib.metadata.entry_points()["dxtbx.profile_model"]:
+        for entry_point in profile_entry_points:
             if entry_point.name == obj["__id__"]:
                 return entry_point.load().from_dict(obj)
         logging.getLogger("dxtbx.model.profile").warn(