Remove Clara from external folder

As Clara is no longer maintained as a separate project, the
implementation was moved to the internal subfolder of top-level
folder. This removes one folder and avoids potential user confusion.

Also simplified the convenience header checking script accordingly.

src/catch2/catch_all.hpp

@@ -36,11 +36,11 @@
 #include <catch2/catch_totals.hpp>
 #include <catch2/catch_version.hpp>
 #include <catch2/catch_version_macros.hpp>
-#include <catch2/external/catch_external_all.hpp>
 #include <catch2/generators/catch_generators_all.hpp>
 #include <catch2/interfaces/catch_interfaces_all.hpp>
 #include <catch2/internal/catch_assertionhandler.hpp>
 #include <catch2/internal/catch_clara.hpp>
+#include <catch2/internal/catch_clara_upstream.hpp>
 #include <catch2/internal/catch_commandline.hpp>
 #include <catch2/internal/catch_common.hpp>
 #include <catch2/internal/catch_compiler_capabilities.hpp>

src/catch2/internal/catch_clara.hpp

@@ -23,7 +23,7 @@
 #pragma clang diagnostic ignored "-Wshadow"
 #endif
 
-#include <catch2/external/clara.hpp>
+#include <catch2/internal/catch_clara_upstream.hpp>
 
 #ifdef __clang__
 #pragma clang diagnostic pop

tools/scripts/checkConvenienceHeaders.py

@@ -13,7 +13,6 @@ The top level header is called `catch_all.hpp`.
 """
 
 internal_dirs = ['detail', 'internal']
-excluded_dirs = ['external']
 
 from scriptCommon import catchPath
 from glob import glob
@@ -44,8 +43,6 @@ def folders_in_folder(folder):
 def collated_includes(folder):
     base = headers_in_folder(folder)
     for subfolder in folders_in_folder(folder):
-        if folder in excluded_dirs:
-            continue
         if subfolder.name in internal_dirs:
             base.extend(headers_in_folder(subfolder.path))
         else:
@@ -140,7 +137,7 @@ def walk_source_folders(current):
     verify_convenience_header(current)
     for folder in folders_in_folder(current.path):
         fname = folder.name
-        if fname not in internal_dirs and fname not in excluded_dirs:
+        if fname not in internal_dirs:
             walk_source_folders(folder)
 
 # This is an ugly hack because we cannot instantiate DirEntry manually