Bug#799121: local repository in non-world-readable directory doesn't work

[Evgeny Kapun <abacabadabacaba@gmail.com>]

Package: apt
Version: 1.1~exp12

I'm using APT with a local repository configured with a file: URI. Also, the directory containing the repository is not world-readable. This works fine with APT 1.0, but not with APT 1.1.
I found that APT 1.1 runs some of its helper programs under a restricted _apt account, and they attempt to access repository files directly and fail. Specifically, when running `apt update`, debug logs indicate that `/usr/lib/apt/methods/xz` program attempts to access repository files and fails with EACCESS, probably as a result of running with limited privileges.