musl-libintl package is a broken approach
Right now we're splitting musl's libintl.h into musl-libintl. This brings along multiple problems:
It conflicts with gettext-dev
libc still provides the gettext symbols. This means that packages checking for those symbols will find them and as such will assume that gettext functionality is built into libc (and as such there's no need to link libintl). See e.g. https://gitlab.freedesktop.org/pulseaudio/pulseaudio/blob/bff163089c5cd629116391cde55f55c73f422d10/meson.build#L299
Most package just end up with linking GNU gettext instead of musl's integrated gettext, which makes the entire thing more expensive than it has to be.
I'd propose just keeping the libintl.h header in musl-dev, moving GNU gettext's libintl header elsewhere (if possible) and starting to enable musl gettext in place where we use GNU gettext right now.