feat(oma-refresh): support custom Dir::Etc::SourceList

[eatradish]

No description provided.

[MingcongBai]

The Option Dir::Etc::SourcesList did not function for refresh:

mingcongbai@JellyFocals [ oma@support-custom-sourceslist ] $ oma -o Dir::Etc::sourcelist=/tmp/a install fish
     INFO 请在弹出的密码框输入密码以授权 oma 执行系统管理操作。
     INFO 如果您无法操作该输入框，请按 Ctrl + C 退出并以管理员权限运行 oma。
     INFO 正在刷新本机软件包数据库 ...
    ERROR 无法找到匹配关键字 fish 的软件包。
    ERROR 您指定的操作未成功完成，请查阅上方输出。
mingcongbai@JellyFocals [ oma@support-custom-sourceslist ] ! oma -o Dir::Etc::sourcelist=/tmp/a refresh
     INFO 请在弹出的密码框输入密码以授权 oma 执行系统管理操作。
     INFO 如果您无法操作该输入框，请按 Ctrl + C 退出并以管理员权限运行 oma。
     INFO 正在刷新本机软件包数据库 ...
  SUCCESS 成功刷新本机软件包数据库。系统各软件包均为最新。

Same goes for install.

[MingcongBai]

My mistake with the previous test, APT in fact uses:

• Dir::Etc::SourceList (/etc/apt/sources.list)
• Dir::Etc::sourceparts (/etc/apt/sources.list.d)

For sources.list settings. I have only overrode the first one and mistaken the output for oma-refresh failing to honour the options. Same went for Dir::Cache::Archives which only handles APT cache, but not the lists, which uses:

Dir::State "var/lib/apt";
Dir::State::lists "lists/";

This is to be handled with another pull request, as discussed with @eatradish.

A tangential bug was also found where the download directories were unconditionally set to 0755 permission, a hack we used to workaround old buggy AOSC OS bashrc configurations. Also terms for another pull request.