CRV2: Missing pallet index causes misleading error message

[Kian Paimani]

When I removed all pallet indices in mod runtime to see if it is supported or not, I get:

error: `System` pallet declaration is missing. Please add this line: `pub type System = frame_system;`
    --> /home/kian/remote-builds/polkadot-sdk_minimal-template/runtime/src/lib.rs:96:5
     |
  96 | mod runtime {
     |     ^^^^^^^

I think it is a good choice to no longer support implicit indices, but the error message should be improve.