Use os.path.lexists when checking for virtualenvs

[mx-moth]

This is a fix for a fairly contrived situation I experience because of my particular development environment. For development, I run projects within a Vagrant virtual machine, with the source code living on my host machine (in project/), shared in to the virtual machine guest. I create a virtual environment on my host machine in this directory (project/venv) so that my editor can do code completion, etc. I run, test, and package the code on the the guest machine. When running pip install -e . or similar commands on the guest, the code within pip fails to detect the virtual environment created on the host machine as a virtual environment, causing the command to delve down through the virtual environment directories despite pip attempting to filter virtual environments out. This makes the pip install -e . command take about a minute to run, compared to the few seconds when the virtual environment is correctly ignored.

pip fails to detect the virtual environment in this particular case, as pip uses os.path.exists() to check that project/venv/bin/python exists. This file does exist, but is a symlink to the python executable provided by the system. On my host, the target file does exist, but in the guest (which may use a different version of Python, or structure its directories differently, or...), the target file does not exist. os.path.exists() returns False for dangling symlinks, causing the virtual environment detection code to fail.

This change uses os.path.lexists() instead of os.path.exists(), which returns True for dangling symlinks.

[xavfernandez]

@timheap a small changelog would be lovely

[mx-moth]

I've added a note to CHANGES.txt, and rebase on latest master.

[xavfernandez]

Thanks a lot ! 👍