By default, foundry expects every library to be a git project. This approach does not work well with SDKs in monorepos (like ours). However, you can also install the dependencies using NPM and then use remappings.txt to point to the SDK. This is a little janky, but it seems to be the recommended approach for this case 🤷
see issue here foundry-rs/foundry#1847
Jayant Krishnamurthy
Ali Behjati
Daniel Chew