convert license headers to SPDX

Includes license review of 3rd-party files, adding missing upstream
headers, and moving the UNSW OZPLB license into its own file rather
than replicating it in the header.

CMake: add CMakeForegroundComplexCommands option

When this option is set, custom_commands that have been passed
USES_TERMINAL_DEBUG will get USES_TERMINAL set which puts them in the
console pool in Ninja. This results in direct access to stdio which
allows for direct logging of build output or the possibility of user
input.

CMake: Refactor helpers to be included on demand

As more helper files get added it becomes more expensive to include them
all in common.cmake. We instead add them to the CMAKE_MODULE_PATH and
allow them to be imported as CMake modules. Adding global include guards
allows them to be imported multiple times without processing every time.

cmake: Add set_break() function for debugging

This will start a cmake repl at this point in the build script within
the same scope to provide debugging and introspection functionality.