moves most third-party packages into /third_party

[cjdb]

Proper use of FetchContent requires sequencing all calls to FetchContent_Declare before any calls to FetchContent_MakeAvailable. This allows dependencies with similar FetchContent calls to coalesce their declarations, so that projects don't end up repeatedly downloading and building identical packages.

We introduce a directory dedicated to third-party packages for two reasons:

1. Moving all of the third-party packages into a single directory improves readability, as it becomes more apparent that calls to FetchContent_Declare and FetchContent_MakeAvailable are correctly sequenced.
2. Providing each third-party package with its own directory allows us to colocate the FindPackage_Declare with any relevant project-local patches. There aren't any at present, so we document how this ought to be done below.

libdwarf and libunwind are still handled in /CMakeLists.txt. Their requirements seem to be substantially more complicated than all other third-party packages, so we'll migrate them into /third_party at a later time.

How to patch a third-party package

Suppose googletest needed to be updated, and requires a local patch so cpptrace can continue to support C++11. To integrate this with FetchContent, we:

1. Make the necessary changes in our local fork of google/googletest.
2. Run git format-patch @{u} -o /path/to/cpptrace/third_party/googletest to generate the patch files directly into /third_party/googletest.
3. Edit /third_party/googletest/CMakeLists.txt like so:

  FetchContent_Declare(
     "${package_name}"
     GIT_REPOSITORY "https://github.com/google/googletest.git"
-    GIT_TAG        58d77fa8070e8cec2dc1ed015d66b454c8d78850 # v1.12.1, last to support C++11
+    GIT_TAG        v1.17.0
+    PATCH_COMMAND git am 0001-makes-foo-c++11-compatible.patch
+                         0002-makes-bar-c++11-compatible.patch
+                         0003-makes-baz-c++11-compatible.patch
     OVERRIDE_FIND_PACKAGE
     SYSTEM
   )

This will apply the patches during the FetchContent process, and because all the FetchContent_Declares come first, it will ensure that the patches aren't clobbered by any dependencies.