Don't overlap the virtualenv salt module with the virtualenv library.
Don't obfuscate the original module name.
Add test case which tests for proper deprecated output based on virtualenv version.
Small code and print's cleanup.
Warn the users if they're using `--distribute` without needing.
Add missing import and instantiate a local logger instance.
Warn and don't pass `--never-download` to `virtualenv` if it's version is `>= 0.10`. Fixes #6030.
Set defaults to `None` instead of `''`.
Allow `extra_search_dir` for `virtualenv.create()` to be passed a list. Fixes #6031.
Add test case to check for proper handling of `--never-download`. Refs #6030.

Also added assertions to check for proper deprecation logging messages output.
Added a test case to check for multiple extra search dirs support. Refs #6031.
Show a deprecation warning if `no_site_packages` is used. Fixes #6027.

If both `no_site_packages` and `system_site_packages` are used, throw an error.
Added the appropriate tests.
Update doc string to make it explicit that it's salt we're mentioning.
`virtualenv` version checking is only done if using `virtualenv` and not `pyvenv`.
Warn about using option not available for `virtualenv` and `pyvenv` respectively.

`False` defaults have been turned into `None` to know if the user is even passing `False` and fail.
Fix the `virtualenv` unittests which rely on it's version changing.
No empty strings as defaults!
Extra care regarding empty strings.
Let us not use a deprecated option ourselves unless for testing the deprecation.
Only `strip()` on strings.