Move dependencies in 'external' directory and pdate git submodules

external/Catch2/.gitignore

@@ -0,0 +1,38 @@
+*.build
+!meson.build
+*.pbxuser
+*.mode1v3
+*.ncb
+*.suo
+Debug
+Release
+*.user
+*.xcuserstate
+.DS_Store
+xcuserdata
+CatchSelfTest.xcscheme
+Breakpoints.xcbkptlist
+UpgradeLog.XML
+Resources/DWARF
+projects/Generated
+*.pyc
+DerivedData
+*.xccheckout
+Build
+.idea
+.vs
+.vscode
+cmake-build-*
+benchmark-dir
+.conan/test_package/build
+.conan/test_package/CMakeUserPresets.json
+bazel-*
+build-fuzzers
+debug-build
+.vscode
+msvc-sln*
+# Currently we use Doxygen for dep graphs and the full docs are only slowly
+# being filled in, so we definitely do not want git to deal with the docs.
+docs/doxygen
+*.cache
+compile_commands.json