Uses custom metadata to exclude chain-specific crates.  
The only concern is that devs who want to use chain-specific crates,
still need to select matching versions numbers. Could possibly be
addresses with chain-specific umbrella crates, but currently it should
be possible to use [psvm](https://github.com/paritytech/psvm).

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>