Why azdev only allow run in virtualenv?

If not install azdev in a virtualenv, it will throw errors:
```
root@098c3f2011fb:/# azdev setup -c -r azure-cli-extensions/
Traceback (most recent call last):
File "/usr/local/bin/azdev", line 11, in
sys.exit(main())
File "/usr/local/lib/python3.6/dist-packages/azdev/__main__.py", line 40, in main
config_dir=get_azdev_config_dir())
File "/usr/local/lib/python3.6/dist-packages/azdev/utilities/config.py", line 24, in get_azdev_config_dir
_, env_name = os.path.splitdrive(get_env_path())
File "/usr/lib/python3.6/posixpath.py", line 138, in splitdrive
p = os.fspath(p)
TypeError: expected str, bytes or os.PathLike object, not NoneType
```