meson: allow wayland-protocols to be used as wrap

We need to get the "internal" variable if we've built wayland-protocols
as a subproject. Wayland-protocols already has al of the infrastructure
in place for this, we just need to make use of it.

This requires a bump to Meson 0.54, which we had discussed in the past
but had no reason to move to.

I have installed the .wrap file and checked it in. Due to upstream API
changes in the wrapdb meson >= 0.59.0 is required to run `meson wrap
install wayland-protocols`, which defeats much the value of being used
as a wrap.