make ERC2981:royaltyInfo public

[Amxx]

Fixes #3304

PR Checklist

• Tests
• Documentation
• Changelog entry

[JulissaDantes]

Looks good!

[mcIovin]

@Amxx and @JulissaDantes - one thought around this (I was the person who submitted this issue...)
If this change gets implemented, the royaltyInfo() function will always be compiled as 'public' (even contracts that inherit ERC2981.sol and override royaltyInfo() will need to declare it as public, rather than 'external.'
This is really useful in order to allow super., however, I don't know if this means the EIP will no longer be strictly adhered to. Unfortunately the EIP (for 2981) has explicit code in it, which uses 'external' rather than 'public'.
Does the fact that the EIP itself uses the word 'external' rather than 'public' matter? I know that the interface Id is calculated from the functions names and arguments, so the visibility of the function won't affect the actual bytes4 of the interface identifier, but how important is it to adhere to the exact coding/wording of the EIP?

[Amxx]

ERC define interfaces which have the function marked external. Making it public doesn't change the interfaceId or the function selector.

You can see that other ERC (20, 721, ...) also define interfaces with external function that end up being implement in a public way.

[mcIovin]

@Amxx - I was hoping that was the case; makes sense.
Thanks again!

[brooklynbagel]

Nothing major, but royaltiInfo should be corrected to royaltyInfo