Use BTreeMap instead of HashMap

[bmwiedemann]

Use BTreeMap instead of HashMap
to have deterministic output ordering
to allow for reproducible builds.

See https://reproducible-builds.org/ for why this is good.

Without this patch, ArchLinux' and openSUSE's identity package varied between builds.

Special thanks go to @kpcyrd for help in debugging this.

[sdroege]

Thanks, I'll include this in the next bugfix release :)

[sdroege]

I wonder if this shouldn't actually use https://docs.rs/indexmap so that the order from the user is preserved

[bmwiedemann]

I did some more testing and found that this patch helps to make more openSUSE package builds fully deterministic: mousai identity eww Fragments fractal gstreamer-plugins-rs contrast

Regarding IndexMap:
users had random ordering before, so my guess is that they do not care much if it is alphabetic or insertion order. And performance is not a concern during build (assuming this code is only used during build).

[sdroege]

It's only used during builds, yes.