wsgi: Rework routing dict (again).

Remove special handling of first folder level.
Generalise CAU/CAS context decision.
Split functionalities further, making each method shorter.
Factorise subpath checks.
Factorise response generation when producing a body.

The resulting data structure, if more verbose than the original one, is
not harder to traverse and more extensible.

caucase/test.py

@@ -1198,6 +1198,17 @@ class CaucaseTest(unittest.TestCase):
         """
         raise ValueError('Some generic exception')
 
+      @staticmethod
+      def _placeholder(_):
+        """
+        Placeholder methods, for when method lookup happens before noticing
+        issues in the query.
+        """
+        raise AssertionError('code should fail before actually calling this')
+
+      getCertificateSigningRequest = _placeholder
+      getCertificate = _placeholder
+
     application = wsgi.Application(DummyCAU(), None)
     def request(environ):
       """
@@ -1226,12 +1237,15 @@ class CaucaseTest(unittest.TestCase):
     })[0], 404)
     self.assertEqual(request({
       'PATH_INFO': '/cau',
+      'REQUEST_METHOD': 'GET',
     })[0], 404)
     self.assertEqual(request({
       'PATH_INFO': '/cau/__init__',
+      'REQUEST_METHOD': 'GET',
     })[0], 404)
     self.assertEqual(request({
       'PATH_INFO': '/cau/does_not_exist',
+      'REQUEST_METHOD': 'GET',
     })[0], 404)
 
     self.assertEqual(request({