Get fallback varname from wrap file after it has been configured
Get fallback varname from wrap file after it has been configured

When _subproject_impl() is called the wrap file could not have been
downloaded form wrapdb yet, it is done when fetching the subproject.
Delay getting the variable name to when we actually need it, at that
point we are sure the wrap file has been downloaded.

mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson