Allow for building wasms from other repos.

This works:
```
~/workspace/odra/casper-trade  ODRA_MODULE=Erc20 cargo build --bin casper_trade_contracts_build_contract --target wasm32-unknown-unknown --release -p casper_trade_contracts
```

Allow adding to `Odra.toml`:
```toml
[[contracts]]
fqn = "Erc20"
```

and this should work:
```bash
odra build -c Erc20
```