Major changes:
1) Instead of skipping undesirable scopes, include them and mark them
with errors. Constants can be extracted into more scopes.
2) Update the tests to call through the "public" API. This caused some
baseline changes.
3) Rename refactoring to "Extract Symbol" for generality.
4) Return a second ApplicableRefactorInfo for constants. Distinguish
the two by splitting the action name.
