Merge fyne-cross

[sdassow]

Direct merge of CLI migration changes from fyne-io/fyne-cross#355 migrate/cli branch at 9aed1ff0dd685ca35b3f6cb10133f73d8a18566f without any changes except imports.

[sdassow]

I think we could drop the architecture commands in favour of the general --os parameter we have already, so we would end up with fyne cross [--os darwin] [package] and without --os the host architecture is being build?
The version command could also be dropped.
Which leaves darwin-sdk-extract command, and otherwise the question about the s3 client code and how to handle it.