For install_external_plugin_bin_to_data and
install_external_plugin_bin_to_arch_data, they were using the
'plugin_target' variable instead of just 'target' for the plugin
directory output location, and they were using 'target' instead of
'additional_target' for the data/binaries they were supposed to be
installing.
This addition allows external cmake modules to include the same modules
that are commonly distributed with the main obs-studio repo: things like
FFmpeg, curl, etc.
This cmake module adds various cmake functions used for generating obs
plugins. This currently may not fully support linux; this may require
more changes on linux due to the nature of the file structure linux
uses.
