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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def _populate_cache(platform, pkg_cache, mount_dir):
    '''
    If a ``pkg_cache`` directory is specified, then use it to populate the
    disk image.
    '''
    if not pkg_cache:
        return
    if not os.path.isdir(pkg_cache):
        return

    if platform == 'pacman':
        cache_dir = '{0}/var/cache/pacman/pkg'.format(mount_dir)

    __salt__['file.mkdir'](cache_dir, 'root', 'root', '755')
    __salt__['file.copy'](pkg_cache, cache_dir, recurse=True, remove_existing=True)