[FR]: write_source_files supports writing filegroups

### What is the current behavior?

`write_source_files` requires you to define file by file. This is not very useful when the input is a filegroup (could be generated) and the files we want to write are unknown.

### Describe the feature

`write_source_files` should support an input `srcs` that can take a file, or a filegroup and write the contents of said filegroup to the source tree.

Start by locating the `write_source_files` entry point and inspect how its `srcs` input currently handles individual files. Confirm how filegroups are represented, then verify that both a file and a filegroup write their contents to the source tree.

Written by the indexing model from the issue text.