server only: test a more standard method of disabling exchange We have an "official" config option from core so lets just use it.

debian: don't delete evolve and topic extensions The debian packaging rules deletes all __init__.py to avoid conflicting with the __init__.py that declares the namespace package. Now that the evolve and topic extensions also live in __init__.py files, we clearly don't want to delete all __init__.py files anymore; only the hgext3rd/__init__.py should be deleted.