externals: avoid importing jinja2 on startup (#14308)

Jinja2 costs a tenth to a few tenths of a second to import, so we should avoid importing it on startup.

- [x] only import jinja2 within functions

lib/spack/spack/modules/common.py

@@ -804,10 +804,11 @@ def write(self, overwrite=False):
 
         # Get the template for the module
         template_name = self._get_template()
+        import jinja2
         try:
             env = tengine.make_environment()
             template = env.get_template(template_name)
-        except tengine.TemplateNotFound:
+        except jinja2.TemplateNotFound:
             # If the template was not found raise an exception with a little
             # more information
             msg = 'template \'{0}\' was not found for \'{1}\''

lib/spack/spack/tengine.py

@@ -5,7 +5,6 @@
 import itertools
 import textwrap
 
-import jinja2
 import llnl.util.lang
 import six
 
@@ -13,9 +12,6 @@
 from spack.util.path import canonicalize_path
 
 
-TemplateNotFound = jinja2.TemplateNotFound
-
-
 class ContextMeta(type):
     """Meta class for Context. It helps reducing the boilerplate in
     client code.
@@ -77,6 +73,10 @@ def make_environment(dirs=None):
         dirs = [canonicalize_path(d)
                 for d in itertools.chain(builtins, extensions)]
 
+    # avoid importing this at the top level as it's used infrequently and
+    # slows down startup a bit.
+    import jinja2
+
     # Loader for the templates
     loader = jinja2.FileSystemLoader(dirs)
     # Environment of the template engine