importer: avoid duplicate module load(!); closes #113.

Amazed this one managed to scrape through for so long. Calling
__import__ from within find_module() was causing the target module, in
this case cookielib, to be loaded *then overwritten* by a subsequent
duplicate load higher in the stack.

The result is that cookielib was loaded twice, and, per usual Python
import semantics, a reference to the partially initialized first
cookielib was installed in sys.modules while its code executed.

At the end of cookielib on 2.x, it imports _LWPCookieJar, which in turn
imports the partially built cookielib from sys.modules, then subclasses
the CookieJar from /that/ module.

Everything is wonderful. Then the call returns back up into the import
mechanism which restarts the entire process -- only this time,
_LWPCookieJar is /not/ reinitialized, so the copy in sys.modules is
still left with types pointing at the old module!

So the duplicate import creates a new CookieJar which is not the base
class of LWPCookieJar. Tada! 3 hours debugging.

This is probably a performance fix in disguise, didn't realize things
were so broken. It may also be a regression elsewhere. Urgently need to
finish the tests.

examples/playbook/hosts

@@ -1,2 +1 @@
-#localhost
-z
+localhost

examples/playbook/issue_113.yml

@@ -1,7 +1,9 @@
-
+---
 
 - hosts: all
+  gather_facts: false
   tasks:
+
   - name: Get auth token
     uri:
       url: "https://httpbin.org/post"
@@ -13,7 +15,7 @@
     register: r_token
     no_log: false
     run_once: true
-    delegate_to: localhost
-
-
+    register: foo
 
+  - assert:
+      that: foo.status == 200

mitogen/core.py

@@ -452,6 +452,16 @@ class Importer(object):
     def __repr__(self):
         return 'Importer()'
 
+    def builtin_find_module(self, fullname):
+        parent, _, modname = fullname.rpartition('.')
+        if parent:
+            path = sys.modules[parent].__path__
+        else:
+            path = None
+        fp, pathname, description = imp.find_module(modname, path)
+        if fp:
+            fp.close()
+
     def find_module(self, fullname, path=None):
         if hasattr(_tls, 'running'):
             return None
@@ -473,7 +483,7 @@ class Importer(object):
                 return None
 
             try:
-                __import__(fullname, {}, {}, [''])
+                self.builtin_find_module(fullname)
                 _v and LOG.debug('%r: %r is available locally', self, fullname)
             except ImportError:
                 _v and LOG.debug('find_module(%r) returning self', fullname)
@@ -483,7 +493,7 @@ class Importer(object):
 
     def _refuse_imports(self, fullname):
         if is_blacklisted_import(self, fullname):
-            raise ImportError('Refused')
+            raise ImportError('Refused: ' + fullname)
 
         f = sys._getframe(2)
         requestee = f.f_globals['__name__']