Grouping the extension access control contracts into one `extensions/` folder

[0xisk]

🧐 Motivation
I think grouping the extension access control contracts would be more readable and organized, as it happens with the token standard contracts.

📝 Details

Current structure:

contracts   
│
└─── access
    │   AccessControl.sol
    │   AccessControlCrossChain.sol     "Extension contract"
    │   AccessControlEnumerable.sol    "Extension contract"
    │   IAccessControlEnumerable.sol   "Extension contract interface"
    │   ...
    └───

Expected structure:

contracts   
│
└─── access
     │
     └─── extensions
     │    │   AccessControlCrossChain.sol     "Extension contract"
     │    │   AccessControlEnumerable.sol    "Extension contract"
     │    │   IAccessControlEnumerable.sol   "Extension contract interface"
     │    └───
     │   AccessControl.sol
     │   ...
     └───

[Amxx]

As discussed in the original PR, this would be a breaking change as far as the import paths are concerned.
Suggest we put that in the 5.0 roadmap

[0xisk]

Thanks, @Amxx I just wanted to log it here as discussed so you can discuss with the team it later milestone.

[balajipachai]

@Amxx any updates, can a PR be submitted for it?