refactor module structure to better organise platform-dependent code
All modules which expose functionality to the user should be moved to
OS-Independent, this avoids duplication of documentation in DCL files. Such modules should additionally contain all platform-independent code. All platform dependent code should be contained in separate modules for each platform starting with a
_. As an example,
System.Directory is already organised in such way.
A description of this module organisation should be included in
STANDARDS.md as well.