move away from directory listings for finding generated crates

[demoray]

As identified in #1479, we do not currently handle deleted/renamed specifications well.

This update addresses this via the following:

• Moves to using parsing cargo_toml to parse services/Cargo.toml to know what crates already exist
• Replaces all uses of list_crate_names with using the results of gen_crates

As a byproduct, this identifies that the previously existing spec that would result in azure_svc_codesigning was removed.

ref: https://github.com/Azure/azure-rest-api-specs/pull/26635