Allow convenient use of librest as a subproject
Allow convenient use of librest as a subproject

Meson allows projects to specify the dependency object when used as
subprojects; this avoids having to know the variable name to import into
the superproject.

See: https://mesonbuild.com/Reference-manual_builtin_meson.html#mesonoverride_dependency

GNOME/librest,GNOME/librest,GNOME/librest