meson: Stop using deprecated way getting path of files

meson: Stop using deprecated way getting path of files

The just released meson 1.3 strongly deprecated a hack we were using, emitting
a noisy warning (the hack basically depended on an implementation detail to
work). Turns out there has been a better way available for a while, I just
hadn't found it. 1.4 added a more convenient approach, but we can't rely on
that.

Reviewed-by: Tristan Partin <tristan@neon.tech>
Discussion: https://postgr.es/m/20231129185053.s6c7f73eg7b4ztfi@awork3.anarazel.de
Backpatch: 16-, where the meson build was added.