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

tests/server/sockfilt.c

@@ -85,9 +85,8 @@
  * it!
  */
 
-
 /* buffer is this excessively large only to be able to support things like
-  test 1003 which tests exceedingly large server response lines */
+   test 1003 which tests exceedingly large server response lines */
 #define BUFFER_SIZE 17010
 
 static bool verbose = FALSE;