gettext upstream has supported this for a very long time (since 0.19.7
via commit b3c2a5a242c36fbbaa0c5b17f975d6c638598a23, released in 2015),
and itstool is (mostly) a legacy of the time before gettext had proper
support for these sorts of use cases.

This is similar to the state of intltool, which is described at
https://wiki.gnome.org/MigratingFromIntltoolToGettext

During the port from autotools to meson, the legacy use of itstool was
faithfully translated to meson in the only way possible: by jumping
through hoops to run ninja inside ninja in order to generate the .mo
files for itstool, because meson's i18n module used a flawed design and
there was no "real" target to create those files, only a .PHONY
run_target which other rules cannot depend on.

Although meson 0.60.0 added support for real targets for the built .mo
files, this changed the rules for output filenames, breaking the script.

But msgfmt does not care, and anyways comes with builtin meson functions
for convenient use with XML files. So let's take this opportunity to
drop legacy dependencies and use the modern, builtin tooling, which
fixes this bug as a side effect.

Fixes #170