meson: don't compile import sources four times

meson: don't compile import sources four times

Use a 'convenience library' to do the compilation once and then link the
objects into all the files that need it. Those files are small, so this probably
doesn't matter too much for speed, but has the advantage that we don't get the
same error four times if something goes wrong.

The library is conditionalized in the same way importd itself, because we
cannot build it without the deps.