The modularity team want to define two headers for a module. One public .h for all functions a structs that are accessible outside the module, and one _internal.h header for functions and structs that are internal only. We have a PoC here that can be used for reference.

This ticket is to:

• Create a proposal to let the team know what we're planning, demonstrate the benefits, address potential tradeoffs
• Expand on the PoC and make sure we've addressed potential pain points and unknowns. Some of these include:
  • How do we minimise backporting issues? Can we make most of the changes in s_all, so on a backport the conflicts are resolved automatically?
  • How do we decide which functions go into the `.h` versus the `private.h`? We want this to still be automatic and handled by `s_prototypes`.
  • Where do the existing `_inline.h` files fit into this? I assume they belong in the .h file, but does this make the size of the file unweildy?
  • Do the files stay in src/include, or do we co-locate them inside the src/module_name/ folders?