Cache env config

This commit adds a new cache to the main config object, specifically for
env config. When env config is retrieved via Config.get_env(), it is
not in general possible to determine whether the env is a packaging
environment or not. Because retrieving standard section config needs to
know which "base" section to inherit from, and this is different for
test envs and packaging envs, we require a separate way to retrieve
previously created env config. At creation time we do reliably know
whether this is a packaging or run env, and we want that to persist (if
it changes, we call clear_env() and recreate it with the new type).

src/tox/config/main.py

@@ -41,6 +41,7 @@ def __init__(  # noqa: PLR0913
 
         self._src = config_source
         self._key_to_conf_set: dict[tuple[str, str, str], ConfigSet] = OrderedDict()
+        self._key_to_env_conf: dict[str, EnvConfigSet] = {}
         self._core_set: CoreConfigSet | None = None
         self.memory_seed_loaders: defaultdict[str, list[MemoryLoader]] = defaultdict(list)
 
@@ -159,18 +160,21 @@ def get_env(
         :param loaders: loaders to use for this configuration (only used for creation)
         :return: the tox environments config
         """
-        section, base_test, base_pkg = self._src.get_tox_env_section(item)
-        return self.get_section_config(
-            section,
-            base=base_pkg if package else base_test,
-            of_type=EnvConfigSet,
-            for_env=item,
-            loaders=loaders,
-        )
+        if item not in self._key_to_env_conf:
+            section, base_test, base_pkg = self._src.get_tox_env_section(item)
+            self._key_to_env_conf[item] = self.get_section_config(
+                section,
+                base=base_pkg if package else base_test,
+                of_type=EnvConfigSet,
+                for_env=item,
+                loaders=loaders,
+            )
+        return self._key_to_env_conf[item]
 
     def clear_env(self, name: str) -> None:
         section, _, __ = self._src.get_tox_env_section(name)
         self._key_to_conf_set = {k: v for k, v in self._key_to_conf_set.items() if k[0] == section.key and k[1] == name}
+        self._key_to_env_conf.pop(name)
 
 
 ___all__ = [