Support for separate source files (revisit)

[VorpalBlade]

With KDE 6 being released but not having KDE 6 on every computer yet (work laptop runs Ubuntu, not Arch) it seems it is time to revisit the question of system-specific .src.ini files. There are complications, and set etc was implemented instead earlier. However, when the differences become large this becomes non-trivial.

On the applying side this is simple: chezmoi templates can handle this.

The harder problem is the adding side: Re-add needs to somehow expand chezmoi templates, especially for the source file which is especially tricky, .chezmoi.sourceFile etc will not be set except when chezmoi is processing things "the normal way".

It seems infeasible to re-implement chezmoi template handling fully. Perhaps we could recognize and patch specific parts before using chezmoi execute-template? This would also make handling of add:hide etc more consistent in theory (though this is not a major problem).