Keep package.metadata in new manifest

[onur]

Docs.rs finally got support for building non-default features but looks like cargo is striping package.metadata.docs.rs section from manifest. Docs.rs is heavily using cargo and getting manifest path with cargo::core::package::Package::manifest_path on multiple occasions.

So it would be great to keep package.metadata in new manifest or maybe we can add a function like manifest_path_orig to Package.

[alexcrichton]

Ah yes definitely a bug and not intentional! I'll look to fixing this soon.

[alexcrichton]

Ok, fix submitted as #4167

[onur]

Thanks!