[m-rev.] for review: split source files into parse_tree_module_srcs

[Julien Fischer]

On Sun, 18 Jul 2021, Zoltan Somogyi wrote:

> Break a source file info parse_tree_module_srcs ...
> 
> ... instead of into raw_compilation_units. Besides being more convenient to
> work on, a parse_tree_module_src encodes in its type a significant number
> of invariants that a raw_compilation unit does not.

That's fine.

Julien.