Spack can be extended with external commands (#8612)

This provides a mechanism to implement a new Spack command in a
separate directory, and with a small configuration change point Spack
to the new command.

To register the command, the directory must be added to the
"extensions" section of config.yaml. The command directory name must
have the prefix "spack-", and have the following layout:

  spack-X/
    pytest.ini #optional, for testing
    X/
	  cmd/
	    name-of-command1.py
	    name-of-command2.py
	    ...
    tests/ #optional
      conftest.py
	  test_name-of-command1.py
    templates/ #optional jinja templates, if needed

And in config.yaml:

  config:
    extensions:
      - /path/to/spack-X

If the extension includes tests, you can run them via spack by adding
the --extension option, like "spack test --extension=X"

lib/spack/llnl/util/lang.py

@@ -12,6 +12,8 @@
 import inspect
 from datetime import datetime, timedelta
 from six import string_types
+import sys
+
 
 # Ignore emacs backups when listing modules
 ignore_modules = [r'^\.#', '~$']
@@ -597,3 +599,33 @@ def __str__(self):
 
     def __repr__(self):
         return repr(self.ref_function())
+
+
+def load_module_from_file(module_name, module_path):
+    """Loads a python module from the path of the corresponding file.
+
+    Args:
+        module_name (str): namespace where the python module will be loaded,
+            e.g. ``foo.bar``
+        module_path (str): path of the python file containing the module
+
+    Returns:
+        A valid module object
+
+    Raises:
+        ImportError: when the module can't be loaded
+        FileNotFoundError: when module_path doesn't exist
+    """
+    if sys.version_info[0] == 3 and sys.version_info[1] >= 5:
+        import importlib.util
+        spec = importlib.util.spec_from_file_location(module_name, module_path)
+        module = importlib.util.module_from_spec(spec)
+        spec.loader.exec_module(module)
+    elif sys.version_info[0] == 3 and sys.version_info[1] < 5:
+        import importlib.machinery
+        loader = importlib.machinery.SourceFileLoader(module_name, module_path)
+        module = loader.load_module()
+    elif sys.version_info[0] == 2:
+        import imp
+        module = imp.load_source(module_name, module_path)
+    return module