vfs: deprecate all old classes in scmutil Now that all vfs class moved to the vfs module, we can deprecate the old one.

util: add a way to issue deprecation warning without a UI object Our current deprecation warning mechanism relies on ui object. They are case where we cannot have access to the UI object. On a general basis we avoid using the python mechanism for deprecation warning because up to Python 2.6 it is exposing warning to unsuspecting user who cannot do anything to deal with them. So we build a "safe" strategy to hide this warnings behind a flag in an environment variable. The test runner set this flag so that tests show these warning. This will help us marker API as deprecated for extensions to update their code.