Merge lp://staging/~brian-murray/apport/bug-1336062 into lp://staging/~apport-hackers/apport/trunk

This is pretty well documented in the attached bug report. I've chosen to test this by pretending that '/bin/true' is provided by mypackage. If we are using the cache from the sandbox then the test will pass, if not the test will fail with the following message.

 $ PYTHON=python3 test/run backend_apt_dpkg.test_get_file_package_uninstalled
Testing local source tree.
Running pep8...
Running pyflakes...
--- Testing backend_apt_dpkg.test_get_file_package_uninstalled ---
test_get_file_package_uninstalled (__main__.T)
get_file_package() on uninstalled packages. ... FAIL

======================================================================
FAIL: test_get_file_package_uninstalled (__main__.T)
get_file_package() on uninstalled packages.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test/test_backend_apt_dpkg.py", line 230, in test_get_file_package_uninstalled
    self.assertEqual(impl.get_file_package('/bin/true', True, cache_dir), 'mypackage')
AssertionError: 'coreutils' != 'mypackage'
- coreutils
+ mypackage

----------------------------------------------------------------------
Ran 1 test in 4.107s

FAILED (failures=1)