Validate PEP 517 backend paths (#19834)
## Summary
Prior to this change, we added `backend-path` entries to `sys.path`
without checking that they existed in the current source tree. When an
in-tree backend was omitted from an sdist, the subsequent wheel build
failed with `ModuleNotFoundError` and suggested that the backend was an
undeclared build dependency.
This validates each `backend-path` entry before resolving or invoking
the backend. Missing directories now produce a direct error explaining
that the configured path does not exist, matching the behavior of PyPA's
`build` frontend.
The regression test builds an sdist with a Flit wrapper in
`backend_dir`, intentionally omits that directory from the sdist, and
verifies the diagnostic during the wheel-from-sdist build.
Closes https://github.com/astral-sh/uv/issues/19771.
