fix(turborepo): API naming and dep graph (#5819)
Previously we had a cycle in our dependencies; this hoists the API
definitions out of API Client into their own crate
`turborepo-vercel-api` so that both `turborepo-vercel-api-mock` and
`turborepo-api-client` can consume them.
This is part of incremental progress for enabling iteration on API.
Closes TURBO-1271
