Fix sdist build for optional path dependencies

[messense]

Fixes #1083 by also bundling optional path dependencies.

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[messense]

This does not work actually, build from sdist fails

$ cargo build
error: failed to load manifest for dependency `polars`

Caused by:
  failed to load manifest for dependency `polars-algo`

Caused by:
  failed to read `/Users/messense/Projects/maturin/dist/polars-0.14.9/local_dependencies/polars/polars-algo/Cargo.toml`

Caused by:
  No such file or directory (os error 2)

[messense]

Because of #1084 (comment), I think we need to somehow add the optional path deps to known_path_deps even if they're not activated by Cargo features.

[messense]

Unfortunately there isn't enough metadata:

"dependencies": [
      {
          "name": "polars-algo",
          "source": null,
          "req": "^0.23.0",
          "kind": null,
          "rename": null,
          "optional": true,
          "uses_default_features": true,
          "features": [],
          "target": null,
          "registry": null,
          "path": "/Users/messense/Projects/polars/polars/polars-algo"
      }
]

and the optional polars-algo dep isn't present at packages, thus we can only get its name and path, but we also need it's id which is unavailable.

[messense]

This reminds me of #720 (comment), we might be able to workaround this by using dependencies instead of the structured dep.

https://github.com/koehlma/maturin/blob/dee86e60b1771055210d04fff5d8509d80a0fe04/src/source_distribution.rs#L146-L175