Archives: Second steps to unify searches of 7z and 7za executables

Add a find7zExecutable() function to avoid duplicated code and to prevent forgotten cases, start using it on some parts of the source code.

Those changes are related to https://phabricator.kde.org/D28416 and https://phabricator.kde.org/D28418

Test plan: The one described in the task Solve some bugs related to archives, remove some duplicated code.

Differential Revision: https://phabricator.kde.org/D28417