generate a small services workspace

[ctaggart]

When working on AutoRust, it is very useful to limit the code generation to a crate or two. It is also helpful to limit the workspace to just the generated crates. That is what this PR does.

cargo run --release -p azure-autorust -- -p azure_svc_blobstorage -p azure_mgmt_vmware

Generates this workspace:

[workspace]
members = [
  "svc/blobstorage",
  "mgmt/vmware",
]

This makes Rust Analyzer much happier.

[rylev]

Looks good!