chore: Improve source tree structure

chore: Improve source tree structure

In preparation for adding support for using dependencies from the system
in addition to bundled or downloaded dependencies, restructure the
source tree so that:

- Third party headers are included without a third_party/ prefix, e.g.
  #include <xxhash.h> instead of #include <third_party/xxhash.h>.
- Ccache headers are included with a ccache/ prefix, e.g. #include
  <ccache/util/string.hpp> instead of #include <util/string.hpp>.

This keeps ccache and other headers separated but in another way than
before.