build: drop explicit curlx from hdr paths, refer headers with curlx/ prefix

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

src/tool_formparse.c

@@ -23,8 +23,6 @@
  ***************************************************************************/
 #include "tool_setup.h"
 
-#include <curlx.h>
-
 #include "tool_cfgable.h"
 #include "tool_msgs.h"
 #include "tool_getparam.h"