index(naming): check duplicate index names

[wenchy]

There are two kinds of indexes:

1. Index
2. OrderededIndex

And they have the same format: (Column1,Column2,...)<ColumnX,ColumnY,...>@IndexName.

In fact, the protoc loader plugin has already checked duplicate index names, see tableauio/loader#127.

But to report errors ahead, we should check duplicate index names on protogen stage, but not only on protoc loader plugin.

[Kybxd]

This is hard to implement since we do not get message descriptors when exporting protos, while the default name of index depends on its message descriptor's name when not explicitly specified.

https://github.com/tableauio/loader/blob/32c922717c5b425703d8c5d444070004a8bebc72/internal/index/descriptor.go#L71-L77