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

First steps in order to remove some duplicated code that is used to find the 7z or 7za executable programs. Additionally, in several places some cases aren't taken into account: when there's only a 7za executable, when there's only a 7z executable, and when there's not a 7z nor 7za executable.

In that commit: move a member function and two objects to a more general class.

Those changes are related to https://phabricator.kde.org/D28417 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/D28416