"./mach clean" fails in case of symlinks.

if the build fails, Running ./mach clean gives the following error:

Removing virtualenv directory: /mnt/sda1/Documents/webpages/rust/servo/python/_virtualenv
Error running mach:

    ['clean']

The error occurred in code that was called by the mach command. This is either
a bug in the called code itself or in the way that mach is calling it.

You should consider filing a bug for this issue.

If filing a bug, please include the full output of mach, including this error
message.

The details of the failure are as follows:

OSError: [Errno 39] Directory not empty: '/mnt/sda1/Documents/webpages/rust/servo/python/_virtualenv/lib/python2.7/site-packages/.libs_cffi_backend'

  File "/mnt/sda1/Documents/webpages/rust/servo/python/servo/build_commands.py", line 762, in clean
    shutil.rmtree(virtualenv_path)
  File "/usr/lib/python2.7/shutil.py", line 270, in rmtree
    rmtree(fullname, ignore_errors, onerror)
  File "/usr/lib/python2.7/shutil.py", line 270, in rmtree
    rmtree(fullname, ignore_errors, onerror)
  File "/usr/lib/python2.7/shutil.py", line 270, in rmtree
    rmtree(fullname, ignore_errors, onerror)
  File "/usr/lib/python2.7/shutil.py", line 270, in rmtree
    rmtree(fullname, ignore_errors, onerror)
  File "/usr/lib/python2.7/shutil.py", line 279, in rmtree
    onerror(os.rmdir, path, sys.exc_info())
  File "/usr/lib/python2.7/shutil.py", line 277, in rmtree
    os.rmdir(path)

Environment: Linux (Ubuntu 19.04)

Start in servo/build_commands.py at the clean command around line 762 and reproduce the failure with ./mach clean on Linux using a virtualenv containing the reported symlink. Check how cleanup handles that path; done means ./mach clean removes the virtualenv without the Directory not empty error.