apt.Cache(rootdir='/') causes PermissionError

[Benjamin Drung]

The apport test case test_run_report_bug_kernel_thread fails in my schroot environment. My schroot environment has only one /etc/apt/sources.list.d/ubuntu.sources file and no /etc/apt/sources.list file.

Running following test script as non-root will fail:

```
#!/usr/bin/python3
import apt
apt.Cache(rootdir='/')
```

$ ./test
Traceback (most recent call last):
  File "./test", line 3, in <module>
    apt.Cache(rootdir='/')
  File "/usr/lib/python3/dist-packages/apt/cache.py", line 143, in __init__
    self._check_and_create_required_dirs(rootdir)
  File "/usr/lib/python3/dist-packages/apt/cache.py", line 186, in _check_and_create_required_dirs
    open(rootdir + f, "w").close()
PermissionError: [Errno 13] Permission denied: '//etc/apt/sources.list'

[Julian Andres Klode (juliank)]

rootdir generally expects a writable directory, but it seems like a nice idea to make it not write a sources.list if sources.list.d exist

[Benjamin Drung (bdrung)]

And apport should use `apt.Cache()` instead of `apt.Cache(rootdir='/')`.