fix ale_python_auto_virtualenv to correctly set virtualenv env vars

[rmuir]

First, thank you for ALE!

According to the documentation, ale_python_auto_virtualenv should automatically set environment variables for commands, but previously the variables were not set completely or correctly.

Before:
PATH variable was expanded to include /path/to/venv
After:
PATH variable is expanded to include /path/to/venv/bin
VIRTUAL_ENV variable is set to path/to/venv

This mimics exactly what the activate scripts do, and allows the configuration knob to work as expected.

For example, after this change, jedi-language-server can be installed globally (instead of inside every venv), and it will "just work" (e.g. find references to dependencies in the venv) when editing a file in a project that uses a venv, because the correct variables are set.

[rmuir]

Example ALEInfo output with this change:

Before:

(started) ['/usr/bin/bash', '-c', 'PATH=''/home/rmuir/workspace/beer/api/.venv''":$PATH" ''jedi-language-server''']

as you can see, PATH is not set correctly as it doesn't include bin directory. the VIRTUAL_ENV variable is missing.

After:

(started) ['/bin/bash', '-c', 'PATH=''/home/rmuir/workspace/beer/api/.venv/bin''":$PATH" VIRTUAL_ENV=''/home/rmuir/workspace/beer/api/.venv'' ''jedi-language-server''']

[rmuir]

Converted to draft until I fix the test to expect the correct data.

[rmuir]

I'll leave it as a separate discussion if the default value for this variable should stay as 0, as I don't want to complicate this PR.

IMO, when working with a python virtualenv, you should always set these variables, same as if the user did activate.

You can sometimes get away with just $env/bin/linter, but not always. So it is best to set the PATH and VIRTUAL_ENV to ensure linter functions correctly. A good example is ansible-lint in a virtualenv, it invokes ansible-playbook --syntax-check which will not work with the current approach, as it won't be in the PATH.

[hsanson]

Looks good to me, thanks.