[FR]: write_source_files supports writing filegroups

[bencoxford]

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.