external-project-helpers: Support multiple calls

If multiple calls to the same target, don't create the same step target
multiple times.

Signed-off-by: Oliver Scott <Oliver.Scott@data61.csiro.au>

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: 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.

style: style cmake files consistently

CMake: Added External Project Helpers

Added a helper file for declaring output object files from an
external project. The helper, 'DeclareExternalProjObjectFiles' is
used to create a custom command that forces stale checks on a given
list of files in the external project.