- The Pods/ directory always exists even before submodules have been
initialized; check for Pods/.git instead, which may be a file or
a folder depending on how the submodule was set up.
- When cloning for the first time, make sure the remote is called
'private' so that later updates will reference it correctly. Also
run `git submodule absorbgitdirs` so that git stores the submodule
state inside the top-level .git directory.
- For an existing directory, simplify the check for the presence of
a proper 'private' remote.
- Check that we're running from the repository root so we don't get
weird errors or do random clones.
- Make the script Shellcheck-clean.
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