Try to create rule from BAG to CURRENT msg

fixes CORE-6641

Consider the following scenario for a message G with the following
versions:

G0 -- rule.bmr --> G1 -- auto-generated rule --> G3

where G3 is the CURRENT msg, and G2 is the BAG message that needs
migration.

We need to check for a migration rule like this first:

G2 --> G3

instead of the backwards ones that were generated before this bug fix:

G2 --> G1

Note that the 'lazy' check to valid the rules in the make_update_rule
method makes that rule pass initially, but later it fails because
there's no rule for the sub fields.

tools/rosbag/src/rosbag/migration.py

@@ -1006,6 +1006,18 @@ def find_path(self, old_class, new_class):
                 found_start = True
                 break
 
+        # Next see if we can create a valid rule directly to the end, including the sub rules
+        if not found_start:
+            new_rule = self.make_update_rule(old_class, new_class)
+            R = new_rule(self, 'GENERATED.' + new_rule.__name__)
+            if R.valid:
+                R.find_sub_paths()
+                if R.sub_rules_valid:
+                    sn = ScaffoldNode(old_class, new_class, R)
+                    self.extra_nodes.append(sn)
+                    self.found_paths[key] = [sn]
+                    return [sn]
+
         # Next see if we can create a valid rule
         if not found_start:
             for (ind, tmp_sn) in reversed(list(zip(range(len(sn_range)), sn_range))):