meson: Stop using fallback: kwarg for deps that don't need it

meson: Stop using fallback: kwarg for deps that don't need it

Subprojects that use meson.override_dependency() do not require the
caller to provide the dependency variable name inside the subproject.

We also don't want to provide the *subproject* name, because the
subproject name can be `pango-1.50.12` instead of `pango` when using
wrap-file to download the tarball instead of using wrap-git. This
causes the pango subproject to be executed twice when using gtk as
a subproject inside gstreamer (which uses pango-1.50.12 as
a wrap-file).

All the dependencies we use can be switched in this way, but the
remaining ones need to be changed to use meson.override_dependency()
first.