Fix new InstrumentationSpec for filegroup

This puts "srcs" under dependency_attributes instead of source_attributes, somewhat counter-intuitively:

1. "srcs" needs to be under dependency_attributes because filegroup just forwards FileProvider without any idea how the files are used. It doesn't know if the dependencies of stuff in srcs are runtime dependencies or not, so that needs to be forwarded indiscriminately and then dealt with by the consumer's InstrumentationSpec.

2. "srcs" needs to not be under source_attributes because source_attributes is for sources that this rule _processes_. filegroup doesn't process anything. (Previously, it needed to include source files it _directly included_ because it might have consumers using that legacy configuration, and in that case those consumers would never gather files from filegroup via FileProvider because it provided InstrumentedFilesProvider.)

RELNOTES: Make filegroup always forward InstrumentedFilesProvider and not collect any sources directly.
PiperOrigin-RevId: 323563516