Because of problems explained in bug 771827, the
code for FileSystemGetter in server/cros/dynamic_suite/control_file_getter.py
is sometimes asked to search all of /usr/local/autotest, which
slows down the world.  Worse, the search code isn't safe in the presence
of symlinks, as described in bug 837388.

Because of recent outages caused by the search code, we need
to do better.  Unfortunately, fixing bug 771827 the right way
could be awkward (the bug has the details).

As it happens, there's an existing hack in the code that blacklists
selected directories:
        # Do not explore site-packages. (crbug.com/771823)
        # Do not explore venv. (b/67416549)
        # (Do not pass Go. Do not collect $200.)
        blacklist = {'site-packages', 'venv'}

The blacklist is conceptually wrong. It's also a maintenance hazard,
since there's nothing to guarantee that the blacklist stays up-to-date
in the face of content changes in /usr/local/autotest.  However, the
blacklist is useful for an immediate fix to prevent the worst of the
recent problems.  So, we should make two changes:
 1) Update the blacklist to contain at least the following:
        site-packages
        venv
        results
        containers
        logs
 2) Update this test:
                    elif os.path.isdir(fullpath):
                        directories.append(fullpath)
    Change it to test `os.path.isdir(fullpath) and not os.path.islink(fullpath)`