spack load/environments: allow customization of prefix inspections (#18260)

`spack load` and `spack env activate` now use the prefix inspections
defined in `modules.yaml`. This allows users to customize/override
environment variable modifications if desired.

If no `prefix_inspections` configuration is present, Spack uses the
values in the default configuration.

lib/spack/spack/user_environment.py

@@ -5,6 +5,7 @@
 import sys
 import os
 
+import spack.config
 import spack.util.prefix as prefix
 import spack.util.environment as environment
 import spack.build_environment as build_env
@@ -25,6 +26,10 @@ def prefix_inspections(platform):
         A dictionary mapping subdirectory names to lists of environment
             variables to modify with that directory if it exists.
     """
+    inspections = spack.config.get('modules:prefix_inspections', None)
+    if inspections is not None:
+        return inspections
+
     inspections = {
         'bin': ['PATH'],
         'lib': ['LD_LIBRARY_PATH', 'LIBRARY_PATH'],