WA2 Extensions had two features that have proven more trouble then they
are worth, and so they were removed from WA3 Plugins:
- Virtual methods (methods that automatically invoked super's version
without having to explicitly do that).
- It used to be possible to use strings or tuples to specify parameters
for extensions, and those were automatically "promoted" to Parameter
instances by the metaclass. This feature was never actually used.
This commit cleans up residual code for these features from Plugin
implementations and removes tests that used to exercise them.
- correct the import for merge_config_values fuction
- fixed expected result for set-with-list case; sequence merge will
produce unique values.
- check that the merge result type matches the other's type
