Avoid confusing modules with executables in bin

When an imported module shares the same name as an executable in the isort script's dir, isort will no longer classify it as a first-party module, even if bin appears before site-packages in sys.path

E.g. this is a problem for modules such as gunicorn (because it also installs a script named "gunicorn"), but only when isort has been installed (if running it out of its source dir, it doesn't see the gunicorn executable)