Renaming to native for c-core root dir

[dennycd]

Change Summary

• renaming native_src to native
• updating Package.swift and source sync script for the folder naming

Test & Verify

run shallow source sync locally to verify

./scripts/sync_grpc_src_shallow.sh 

verify spm build via

swift build && swift test

[sampajano]

This PR is too big and i cannot open it on Github to give LGTM.. so.. LGTM by comment (if it's just moving things).. 😂