[build] use git clone instead of submodules

[PiRK]

Git submodules are clumsy to use. We can achieve the same result with a simple git clone followed by a git checkout <commit hash>.

This will also avoid headaches regarding license compatibility if we ever move Electrum ABC into the Bitcoin ABC mono-repo.

TODO:

• [x] electrum-locale
• [x] do some caching to avoid cloning unnecessarily

• [x] bail out early in the script if the repository is already present, instead of failing (check that the binary has been compiled, first)

  All the seemingly unrelated commits in this PR are necessary to fix the CI builds and be able to properly test the main change, except for 5029b40 and 53a289a which are backports from electrum that simplify the build scripts.

[PiRK]

Current status :

• I'm still working on fixing an issue with the windows build after bumping the python version. pyscard 2.0.0 did not have a whl the last time contrib/freeze_packages was executed, so the hash for the ...cp39-win32.whl is missing. The easy fix is to just add this hash to deterministic-build/requirements-hw.txt, but first I want to try to fix the issue by rerunning freeze-packages to update everything and see if more issues appear with recent packages.
• I still cannot test the MacOS build because Cirrus CI stopped supporting the Catalina-base image, and the build does not work on more recent versions (Big Sure, ...) because zbar won't compile. WIP.

[PiRK]

Note: ARM support still does not work yet. c323a64 is only a partial fix.