build: fix libcurltool with cmake and tunits, related tidy-ups

Sync how libcurltool is built in different modes and build systems.

cmake:
- build libcurltool with curlx when building shared libcurl.
  To make it possible to use standard libcurl when linking tunits.
  Also syncing this with autotools.
  The remaining difference is that cmake allows to select shared or
  static for curl tool and tests/examples independently.
- fix to link with libcurl instead of libcurlu.
  To sync with autotools and to link with the standard libcurl for
  tool unit tests.
- fix `source_group()` to always include curlx sources.
- add missing 'curlx header files' source group.

autotools:
- build libcurltool without curlx when building static libcurl in
  non-unity builds.
  To avoid double compilation, just to be thrown away at link time.
  Also to sync with unity builds.

both:
- sync source order between autotools and cmake.
- make sure to pass all headers with both autotools and cmake.
  This is a no-op with cmake. Maybe a future patch should make sure
  to not pass those to remove that noise.

Ref: #17696

Closes #17727

src/Makefile.inc

@@ -150,6 +150,3 @@ CURL_HFILES = \
   var.h
 
 CURL_RCFILES = curl.rc
-
-# curl_SOURCES is special and gets assigned in src/Makefile.am
-CURL_FILES = $(CURL_CFILES) $(CURLX_CFILES) $(CURL_HFILES)