Standardize header names and file locations

This is both a really big and a really small commit. It is small in
that it only contains renaming, moving and modification of include
directives caused by this.

It is really big in the obvious way of touching something like 200
files.

The new rules for naming files is simple: headers use the `.hpp`
extension. The rules for physical file layout is still kinda in
progress, but the basics are also simple:
 * Significant parts of functionality get their own subfolder
   * Benchmarking is in `catch2/benchmark`
   * Matchers are in `catch2/matchers`
   * Generators are in `catch2/generators`
   * Reporters are in `catch2/reporters`
   * Baseline testing facilities are in `catch2/`
 * Various top level folders also contain `internal` subfolder,
   with files that users probably do not want to include directly,
   at least not until they have to write something like their own
   reporter.
    * The exact files in these subfolders is likely to change later
      on

Note that while some includes were cleaned up in this commit, it
is only the low hanging fruit and further cleanup using automatic
tooling will happen later.

Also note that various include guards, copyright notices and file
headers will also be standardized later, rather than in this commit.

src/catch2/internal/catch_debugger.cpp

@@ -0,0 +1,123 @@
+/*
+ *  Created by Phil on 27/12/2010.
+ *  Copyright 2010 Two Blue Cubes Ltd. All rights reserved.
+ *
+ *  Distributed under the Boost Software License, Version 1.0. (See accompanying
+ *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ *
+ */
+
+#include <catch2/internal/catch_debugger.hpp>
+#include <catch2/internal/catch_errno_guard.hpp>
+#include <catch2/internal/catch_stream.hpp>
+#include <catch2/internal/catch_platform.hpp>
+
+#if defined(CATCH_PLATFORM_MAC) || defined(CATCH_PLATFORM_IPHONE)
+
+#  include <assert.h>
+#  include <stdbool.h>
+#  include <sys/types.h>
+#  include <unistd.h>
+#  include <cstddef>
+#  include <ostream>
+
+#ifdef __apple_build_version__
+    // These headers will only compile with AppleClang (XCode)
+    // For other compilers (Clang, GCC, ... ) we need to exclude them
+#  include <sys/sysctl.h>
+#endif
+
+    namespace Catch {
+        #ifdef __apple_build_version__
+        // The following function is taken directly from the following technical note:
+        // https://developer.apple.com/library/archive/qa/qa1361/_index.html
+
+        // Returns true if the current process is being debugged (either
+        // running under the debugger or has a debugger attached post facto).
+        bool isDebuggerActive(){
+            int                 mib[4];
+            struct kinfo_proc   info;
+            std::size_t         size;
+
+            // Initialize the flags so that, if sysctl fails for some bizarre
+            // reason, we get a predictable result.
+
+            info.kp_proc.p_flag = 0;
+
+            // Initialize mib, which tells sysctl the info we want, in this case
+            // we're looking for information about a specific process ID.
+
+            mib[0] = CTL_KERN;
+            mib[1] = KERN_PROC;
+            mib[2] = KERN_PROC_PID;
+            mib[3] = getpid();
+
+            // Call sysctl.
+
+            size = sizeof(info);
+            if( sysctl(mib, sizeof(mib) / sizeof(*mib), &info, &size, nullptr, 0) != 0 ) {
+                Catch::cerr() << "\n** Call to sysctl failed - unable to determine if debugger is active **\n" << std::endl;
+                return false;
+            }
+
+            // We're being debugged if the P_TRACED flag is set.
+
+            return ( (info.kp_proc.p_flag & P_TRACED) != 0 );
+        }
+        #else
+        bool isDebuggerActive() {
+            // We need to find another way to determine this for non-appleclang compilers on macOS
+            return false;
+        }
+        #endif
+    } // namespace Catch
+
+#elif defined(CATCH_PLATFORM_LINUX)
+    #include <fstream>
+    #include <string>
+
+    namespace Catch{
+        // The standard POSIX way of detecting a debugger is to attempt to
+        // ptrace() the process, but this needs to be done from a child and not
+        // this process itself to still allow attaching to this process later
+        // if wanted, so is rather heavy. Under Linux we have the PID of the
+        // "debugger" (which doesn't need to be gdb, of course, it could also
+        // be strace, for example) in /proc/$PID/status, so just get it from
+        // there instead.
+        bool isDebuggerActive(){
+            // Libstdc++ has a bug, where std::ifstream sets errno to 0
+            // This way our users can properly assert over errno values
+            ErrnoGuard guard;
+            std::ifstream in("/proc/self/status");
+            for( std::string line; std::getline(in, line); ) {
+                static const int PREFIX_LEN = 11;
+                if( line.compare(0, PREFIX_LEN, "TracerPid:\t") == 0 ) {
+                    // We're traced if the PID is not 0 and no other PID starts
+                    // with 0 digit, so it's enough to check for just a single
+                    // character.
+                    return line.length() > PREFIX_LEN && line[PREFIX_LEN] != '0';
+                }
+            }
+
+            return false;
+        }
+    } // namespace Catch
+#elif defined(_MSC_VER)
+    extern "C" __declspec(dllimport) int __stdcall IsDebuggerPresent();
+    namespace Catch {
+        bool isDebuggerActive() {
+            return IsDebuggerPresent() != 0;
+        }
+    }
+#elif defined(__MINGW32__)
+    extern "C" __declspec(dllimport) int __stdcall IsDebuggerPresent();
+    namespace Catch {
+        bool isDebuggerActive() {
+            return IsDebuggerPresent() != 0;
+        }
+    }
+#else
+    namespace Catch {
+       bool isDebuggerActive() { return false; }
+    }
+#endif // Platform