stage : try to remove dead links only of folder that you actually care about

A use case where the previous approach was failing is :

 - more than one spack process running on compute nodes
 - stage directory is a link to fast LOCAL storage

 In this case the processes may try to unlink something that is "dead" for them, but actually used by other processes on storage they cannot see.

lib/spack/llnl/util/filesystem.py

@@ -39,15 +39,34 @@
 import llnl.util.tty as tty
 from llnl.util.lang import dedupe
 
-__all__ = ['set_install_permissions', 'install', 'install_tree',
-           'traverse_tree',
-           'expand_user', 'working_dir', 'touch', 'touchp', 'mkdirp',
-           'force_remove', 'join_path', 'ancestor', 'can_access',
-           'filter_file',
-           'FileFilter', 'change_sed_delimiter', 'is_exe', 'force_symlink',
-           'set_executable', 'copy_mode', 'unset_executable_mode',
-           'remove_dead_links', 'remove_linked_tree',
-           'fix_darwin_install_name', 'find_libraries', 'LibraryList']
+__all__ = [
+    'FileFilter',
+    'LibraryList',
+    'ancestor',
+    'can_access',
+    'change_sed_delimiter',
+    'copy_mode',
+    'expand_user',
+    'filter_file',
+    'find_libraries',
+    'fix_darwin_install_name',
+    'force_remove',
+    'force_symlink',
+    'install',
+    'install_tree',
+    'is_exe',
+    'join_path',
+    'mkdirp',
+    'remove_dead_links',
+    'remove_if_dead_link',
+    'remove_linked_tree',
+    'set_executable',
+    'set_install_permissions',
+    'touch',
+    'touchp',
+    'traverse_tree',
+    'unset_executable_mode',
+    'working_dir']
 
 
 def filter_file(regex, repl, *filenames, **kwargs):
@@ -388,10 +407,20 @@ def remove_dead_links(root):
     """
     for file in os.listdir(root):
         path = join_path(root, file)
-        if os.path.islink(path):
-            real_path = os.path.realpath(path)
-            if not os.path.exists(real_path):
-                os.unlink(path)
+        remove_if_dead_link(path)
+
+
+def remove_if_dead_link(path):
+    """
+    Removes the argument if it is a dead link, does nothing otherwise
+
+    Args:
+        path: the potential dead link
+    """
+    if os.path.islink(path):
+        real_path = os.path.realpath(path)
+        if not os.path.exists(real_path):
+            os.unlink(path)
 
 
 def remove_linked_tree(path):