To make all src and test code refer to curlx headers the same way.
Also:
- src: move `curlx.h` include to `tool_setup.h`.
- src/tool_setup.h: drop stray `curlx/timeval.h`.
- servers: de-duplicate `curlx.h` and `curl_setup.h` includes.
- libtests, units: drop stray curlx sub-headers in favor of
`<curlx/curlx.h>`.
- tests: include `curlx.h` with `<>` instead of `""`. To match
other parts of the codebase.
Closes#17680
1a70977a83