Problem: Installing plugin with submodules doesn't check out their
state (due to `git clone --no-checkout` to not end up with default
branch code in case of invalid `version`).
Updating a plugin with submodules doesn't update their state.
Solution: Update `git_checkout` helper to account for submodules.
Another approach would be `git checkout --recurse-submodules ...`,
but that doesn't seem to allow `--filter=blob:none` for submodules,
which is nice to have.
Also make `git_clone` wrapper simpler since `--no-checkout` makes
`--recurse-submodules` and `--also-filter-submodules` do nothing.
bac4cde9cd