Symlink chain-specs json files to crate where they are used (#1171)

When publishing crates, each crate becomes it's own tarball that can't
access files from other crates. So symlink the files to be crate local
and cargo will replace the symlinks with real files at publish time.

We can't just move all of them as it makes the package larger than the
max crates.io package size.

Co-authored-by: Javier Viola <[email protected]>