Header files are now self-contained, which makes the relationships
between the files clearer, and crucially allows LSP tools like `clangd`
to function correctly in all of our header files. I have verified that
the headers are self-contained (aside from the various Windows shims) by
compiling them as if they were C files – in a follow-up commit I plan to
add this to CI to ensure we don't regress on this front.
This is a prerequisite to achieving self-contained headers. Previously,
the various tsd implementation headers (`tsd_generic.h`,
`tsd_tls.h`, `tsd_malloc_thread_cleanup.h`, and `tsd_win.h`) relied
implicitly on being included in `tsd.h` after a variety of dependencies
had been defined above them. This commit instead makes these
dependencies explicit by splitting them out into a separate file,
`tsd_internals.h`, which each of the tsd implementation headers includes
directly.
