lib: use lib source directory as base include path

Backtrack on previous change that aimed to solve the wrong `share.h`
being included. It turns out it did not fix this issue. At the same time
it introduced relative header filenames and the need to include the same
headers differently depending on the source files' location, reducing
readability and editability.

Replace this method by re-adding curl's lib source directory to the
header path and addressing headers by the their full, relative name to
that base directory. Aligning with this method already used in src and
tests.

With these advantages:
- makes includes easier to read, recognize, grep, sort, write, and copy
  between sources,
- syncs the way these headers are included across curl components,
- avoids the ambiguity between system `schannel.h`, `rustls.h` vs.
  local headers using the same names in `lib/vtls`,
- silences clang-tidy `readability-duplicate-include` checker, which
  detects the above issue,
  Ref: https://clang.llvm.org/extra/clang-tidy/checks/readability/duplicate-include.html
- possibly silences TIOBE coding standard warnings:
  `6.10.2.a: Don't use relative paths in #include statements.`
- long shot: it works well with concatenated test sources, for
  clang-tidy-friendly custom unity builds. Ref: #20667

Slight downside: it's not enforced.

If there happens to be a collision between a local `lib/*.h` header and
a system one, the solution is to rename (possibly with its `.c`
counterpart) into the `curl_` namespace. This is also the method used by
curl in the past.

Also:
- curlx/inet_pton: reduce scope of an include.
- toolx/tool_time: apply this to an include, and update VS project
  files accordingly. Also dropping unnecessary lib/curlx header path.
- clang-tidy: enable `readability-duplicate-include`.

Follow-up to 3887069c66 #19676
Follow-up to 625f2c1644 #16991 #16949

Closes #20623

lib/vtls/x509asn1.c

@@ -21,7 +21,7 @@
  * SPDX-License-Identifier: curl
  *
  ***************************************************************************/
-#include "../curl_setup.h"
+#include "curl_setup.h"
 
 #if defined(USE_GNUTLS) || defined(USE_WOLFSSL) || defined(USE_SCHANNEL) || \
   defined(USE_MBEDTLS) || defined(USE_RUSTLS)
@@ -36,12 +36,12 @@
 #define WANT_EXTRACT_CERTINFO /* uses Curl_extract_certinfo() */
 #endif
 
-#include "../urldata.h"
-#include "vtls.h"
-#include "../curl_trc.h"
-#include "../curlx/base64.h"
-#include "x509asn1.h"
-#include "../curlx/dynbuf.h"
+#include "urldata.h"
+#include "vtls/vtls.h"
+#include "curl_trc.h"
+#include "curlx/base64.h"
+#include "vtls/x509asn1.h"
+#include "curlx/dynbuf.h"
 
 /*
  * Constants.