[FR]: write_source_files and write_source_file shall support select()

### What is the current behavior?

write_source_files and write_source_file are implemented as macros therefor select() cannot be used.

### Describe the feature

Reimplement write_source_files and write_source_file as pure rules that can benefit from select()

Start by locating the definitions and tests for write_source_files and write_source_file in the repository. Check how their current macro behavior prevents select() and define the expected behavior for pure rules; done means both entry points support select() without regressing existing use cases.

Written by the indexing model from the issue text.