locks: fix bug when creating lockfiles in the current directory.

- Fixes a bug in `llnl.util.lock`

- Locks in the current directory would fail because the parent directory
  was the empty string.

- Fix this and return '.' for the parent of locks in the current
  directory.

lib/spack/llnl/util/lock.py

@@ -150,6 +150,11 @@ def _lock(self, op, timeout=_default_timeout):
 
     def _ensure_parent_directory(self):
         parent = os.path.dirname(self.path)
+
+        # relative paths to lockfiles in the current directory have no parent
+        if not parent:
+            return '.'
+
         try:
             os.makedirs(parent)
         except OSError as e: