Drop optional dependencies of Spack (#43081)

Remove dependency on `importlib_metadata` and `pkg_resources`, which can be problematic if the version in PYTHONPATH is incompatible with the interpreter Spack is running under.

lib/spack/llnl/util/lang.py

@@ -12,7 +12,6 @@
 import re
 import sys
 import traceback
-import warnings
 from datetime import datetime, timedelta
 from typing import Any, Callable, Iterable, List, Tuple
 
@@ -847,43 +846,25 @@ def __repr__(self):
 def get_entry_points(*, group: str):
     """Wrapper for ``importlib.metadata.entry_points``
 
-    Adapted from https://github.com/HypothesisWorks/hypothesis/blob/0a90ed6edf56319149956c7321d4110078a5c228/hypothesis-python/src/hypothesis/entry_points.py
-
     Args:
-        group (str): the group of entry points to select
+        group: entry points to select
 
     Returns:
-        EntryPoints for ``group``
-
+        EntryPoints for ``group`` or empty list if unsupported
     """
 
     try:
-        try:
-            from importlib import metadata as importlib_metadata  # type: ignore  # novermin
-        except ImportError:
-            import importlib_metadata  # type: ignore  # mypy thinks this is a redefinition
-        try:
-            entry_points = importlib_metadata.entry_points(group=group)
-        except TypeError:
-            # Prior to Python 3.10, entry_points accepted no parameters and always
-            # returned a dictionary of entry points, keyed by group.  See
-            # https://docs.python.org/3/library/importlib.metadata.html#entry-points
-            entry_points = importlib_metadata.entry_points().get(group, [])
-        yield from entry_points
+        import importlib.metadata  # type: ignore  # novermin
     except ImportError:
-        # But if we're not on Python >= 3.8 and the importlib_metadata backport
-        # is not installed, we fall back to pkg_resources anyway.
-        try:
-            import pkg_resources  # type: ignore
-        except ImportError:
-            warnings.warn(
-                "Under Python <= 3.7, Spack requires either the importlib_metadata "
-                "or setuptools package in order to load extensions via entrypoints.",
-                ImportWarning,
-            )
-            yield from ()
-        else:
-            yield from pkg_resources.iter_entry_points(group)
+        return []
+
+    try:
+        return importlib.metadata.entry_points(group=group)
+    except TypeError:
+        # Prior to Python 3.10, entry_points accepted no parameters and always
+        # returned a dictionary of entry points, keyed by group.  See
+        # https://docs.python.org/3/library/importlib.metadata.html#entry-points
+        return importlib.metadata.entry_points().get(group, [])
 
 
 def load_module_from_file(module_name, module_path):