build: un-absorb git submodules before doing OCI build

[tycho]

The worktree paths confuse the Docker builder, so replace the .git submodule stubs with the actual .git trees, without core.worktree set.

[bleggett]

submodules are annoying, but I think if you just remove all the git commands from package.sh you don't need all this.

the git clean/git resets inside the docker image aren't needed anyway, and the git tag thing at the end of package.sh can be made optional via arg (or the value passed in as an arg)

I'll approve, this also works, implementer's choice, it's just a little more wonky (to me, I wasn't aware of absorb)