Revert "snippets: merge c and chdr snippets together when reloading."

This reverts commit 02a9cfeb2cb13a147dfb229ca7042f73aa133a24.
snippets: preload c as chdr with chdr overlay.

This allows us to use things like "g_return_if_fail()" even in headers.
It also allows us to override snippets like "gobject" in headers with
different content.