Suppress virtualenv warning messages on Windows

Suppress 'WARNING: Attempting to work in a virtualenv...' message
on Windows in bash and PowerShell.  Adapts
pip.locations.running_under_virtualenv for use in no sys.executable
symlinks case.
Apply os.path.normcase to venv detection
Add local variable for normalized virtualenv path
Merge pull request #6020 from etgalloway/bash-windows-venv-warning

Disable virtualenv warning in bash on Windows