Expose extension interface in shell API

Tihs is needed in the subunit2sql worker that processes subunit streams.
It doesn't use main(), so we need to be able to load extensions this
way.

Change-Id: Id16c87382f3767982ce815d129ae56a941375546
Implements: counter-inspection

subunit2sql/shell.py

@@ -181,18 +181,27 @@ def process_results(results):
     session.close()
 
 
-def main():
-    cli_opts()
-
+def get_extensions():
     def check_enabled(ext):
         return ext.plugin.enabled()
-    extensions = enabled.EnabledExtensionManager('subunit2sql.target',
-                                                 check_func=check_enabled)
-    parse_args(sys.argv)
+    return enabled.EnabledExtensionManager('subunit2sql.target',
+                                           check_func=check_enabled)
+
+
+def get_targets(extensions):
     try:
         targets = list(extensions.map(lambda ext: ext.plugin()))
     except RuntimeError:
         targets = []
+    return targets
+
+
+def main():
+    cli_opts()
+
+    extensions = get_extensions()
+    parse_args(sys.argv)
+    targets = get_targets(extensions)
     if CONF.subunit_files:
         if len(CONF.subunit_files) > 1 and CONF.run_id:
             print("You can not specify a run id for adding more than 1 stream")