2023-09-26-af86685: #2012: Do not pull non-essential submodules by default.

Only yosys and surelog submodules are needed for plugin build. Those two are automatically fetched as before, i.e. after calling just git submodule update --recursive.

All other submodules are skipped, even when their paths are explicitly specified. To fetch them one has to add --checkout flag has to git submodule update, e.g.:

git submodule update --dept 1 --init --recursive --checkout

Fixes #1962