Use PEP 517 to extract non-static `pyproject.toml` metadata (#2633)
## Summary
When a user passes a `pyproject.toml` to `pip compile` (e.g., `uv pip
compile pyproject.toml`), we extract the requirements from the
`pyproject.toml` directly. However... that isn't always possible (as
seen in the linked issues). When it's _not_, we instead need to run the
PEP 517 build hooks to identify the metadata.
Closes https://github.com/astral-sh/uv/issues/1624.
Closes https://github.com/astral-sh/uv/issues/1644.
## Test Plan
`cargo test`
