Fix module loads (#5599)

Fixes #5455

All methods within setup_package use an EnvironmentModifications object
to control the environment. Those modifications are applied at the end
of setup_package. Module loads for the build environment need to be
done after the rest of the environment modifications are applied, as
otherwise Spack may unset variables set by those modules (for example
LD_LIBRARY_PATH).

lib/spack/spack/build_environment.py

@@ -167,12 +167,6 @@ def set_compiler_environment_variables(pkg, env):
 
     env.set('SPACK_COMPILER_SPEC', str(pkg.spec.compiler))
 
-    for mod in compiler.modules:
-        # Fixes issue https://github.com/LLNL/spack/issues/3153
-        if os.environ.get("CRAY_CPU_TARGET") == "mic-knl":
-            load_module("cce")
-        load_module(mod)
-
     compiler.setup_custom_environment(pkg, env)
 
     return env
@@ -312,9 +306,6 @@ def set_build_environment_variables(pkg, env, dirty):
             if os.path.isdir(pcdir):
                 env.prepend_path('PKG_CONFIG_PATH', pcdir)
 
-    if pkg.architecture.target.module_name:
-        load_module(pkg.architecture.target.module_name)
-
     return env
 
 
@@ -484,7 +475,7 @@ def setup_package(pkg, dirty):
     set_compiler_environment_variables(pkg, spack_env)
     set_build_environment_variables(pkg, spack_env, dirty)
     pkg.architecture.platform.setup_platform_environment(pkg, spack_env)
-    load_external_modules(pkg)
+
     # traverse in postorder so package can use vars from its dependencies
     spec = pkg.spec
     for dspec in pkg.spec.traverse(order='post', root=False, deptype='build'):
@@ -511,6 +502,21 @@ def setup_package(pkg, dirty):
     validate(spack_env, tty.warn)
     spack_env.apply_modifications()
 
+    # All module loads that otherwise would belong in previous functions
+    # have to occur after the spack_env object has its modifications applied.
+    # Otherwise the environment modifications could undo module changes, such
+    # as unsetting LD_LIBRARY_PATH after a module changes it.
+    for mod in pkg.compiler.modules:
+        # Fixes issue https://github.com/LLNL/spack/issues/3153
+        if os.environ.get("CRAY_CPU_TARGET") == "mic-knl":
+            load_module("cce")
+        load_module(mod)
+
+    if pkg.architecture.target.module_name:
+        load_module(pkg.architecture.target.module_name)
+
+    load_external_modules(pkg)
+
 
 def fork(pkg, function, dirty):
     """Fork a child process to do part of a spack build.