Enable building contract with metadata from another contract

[cmichi]

Needed for use-ink/ink#739.

[xgreenx]

I only want to clarify, that this feature is not needed for upgradability=)
Because of the idea of an upgradable contract, that you can change the code at any time, so old ABI can be incorrect, and better to not apply ABI at all for contracts like Proxy.

I think it is better to have some util or command(in cargo-contract) that allows merging several contracts into one ABI file. After, the user can use this ABI file to do a call to the Proxy contract(so he should select the address of the proxy contract and then apply his ABI in UI).

[xgreenx]

Processing metadata from child crates is important in terms of use-ink/ink#809.
So I did an investigation and found the way, how you can collect metadata from crates.
https://github.com/xgreenx/ink-metadata

I created an example of how it can be implemented=) I think it should resolve our problem to generate metadata correct=)

[cmichi]

I'll close this issue as I used the flag mostly for testing and no longer think it makes sense in master.