virtualenvwrapper logic improved

[KenMercusLai]

1. set ENV_NAME to current folder name instead of an empty string when .venv file is not found
2. optimized activate & deactivated logic

[apjanke]

This seems like a substantial change to the existing behavior of virtualenvwrapper, which treats only git repo roots or .venv-customized dirs as project roots.

Conflicts with PR #4122, which fixes a regression related to treating git repo roots as project roots. Could you have a look at that discussion and comment there? At a minimum, this will probably need to be rebased after #4122 is merged. And the change in behavior should probably be put up for discussion with other virtualenvwrapper users.

[apjanke]

Now that #4122 is merged, can you review to see if this is still needed, and if so, rebase it against the latest master? This will probably require some manual changes even if it is a clean rebase, since #4122 included a change to the search logic.