meson: use a convenience library for the sources shared between core/ and the outside

meson: use a convenience library for the sources shared between core/ and the outside

This avoids double compilation. Those files are tiny, so it doesn't save time,
but we avoid repeated warnings and errors, and it's generally cleaner to it
this way.

The number of commands in 'ninja -C build clean && ninja -C build' drops from
1462 to 1455 for me.