Parallelize independent clones?

[robertd-sifive]

For most workspaces the calls to git clone could be done in parallel, at least at the top level of the dependency tree, and it would greatly speed up initializing a complex workspace. It seems like this method would need to be reworked to support this: https://github.com/sifive/wit/blob/master/lib/wit/workspace.py#L180.

[mmjconolly]

As documentation to whoever tries this, you'll also need to rework lib/wit/dependency.py resolve_deps() as it also causes clones.

Some packages have overlapping dependencies, so care would need to be taken such that only one clone is attempted per resulting git repo

[mmjconolly]

Somewhat related is #211 which does a parallel clone

[mmjconolly]

Another attempt at adding parallelism: #222