CI should cache submodules?

[smarter]

The first thing the CI does on every build is git submodule update --init, this takes 23 seconds, most of that time is spent checking out lampepfl/scala twice (it's dumb that git doesn't realize it's doing the same thing twice here). It would be nice if we could cache this. It may also be worth merging scala-backend and scala2-library in a single submodule just to make it easier on everyone trying out dotty (on a bad connection a recursive clone might take a looong time).

[smarter]

@allanrenucci Do you think this is doable or should we close?

[allanrenucci]

I'll will experiment with this over the next week. I think there are simple optimisations I can try such as cloning the submodules in parallel.

Regarding caching, I don't see an easy solution

[allanrenucci]

Closing as I don't plan to implement caching for submodules since I minimised the time spent cloning submodules by doing it in parallel. (ref/ #3850)