Support recursive extras in direct `pyproject.toml` files (#1990)
## Summary
When a `pyproject.toml` is provided directly to `uv pip compile`, we
were failing to resolve recursive extras. The solution I settled on here
is to flatten them recursively when determining the requirements
upfront.
Closes https://github.com/astral-sh/uv/issues/1987.
## Test Plan
`cargo test`
