Simplify how extra files by attributes are collect in incremental generation mode

[brentleyjones]

Having an XcodeProjAutomaticTargetProcessingInfo.extra_files makes it clear that we are only looking at those attributes in order to collect them. e.g. hdrs isn’t used for anything in incremental generation mode, so it doesn’t make sense to have it be a named attribute.

This will also be a memory savings once we drop legacy generation mode.