Add to_list() method to include_directories objects

[WillAyd]

This revives PR #13035 while retaining authorship to @gabeblack

I did not see any outstanding comments so it is the same code as before. I am happy to make any edits that are required

[bonzini]

Fixes #5468

[joukewitteveen]

I think to_list methods should take two optional arguments: start and stop. Since subscripting does not support any form of slicing, the additional methods enable a use-case not otherwise supported. Implementation shouldn't be too hard. For custom_target it could be something like

        start = kwargs.get('start')  # defaults to 0
        stop = kwargs.get('stop', len(self.held_object.outputs))
        selection = range(start, stop)
        return [x for i, x in enumerate(self.held_object) if i in selection]

edit: or simply return list(self.held_object)[kwargs.get('start'):kwargs.get('stop')].

This would enable

headers = ['multiple', 'files']
sources = ['several', 'sources']
custom_outputs = custom_target('example', ..., output : [headers, sources])
just_headers = custom_outputs.to_list(stop=headers.length())

For include_directories, there is a weird quirk in that every input results in two outputs, so you'd need to multiply the start/stop arguments by 2 either in meson.build or in the specific to_list implementation.