msubprojects: if fetching the remote fails, gracefully fallback to local copy
msubprojects: if fetching the remote fails, gracefully fallback to local copy

This command is useful to e.g. update a cloned subproject which does not
have its packagefiles merged, or which has updated packagefiles. This
does not strictly require internet, so if we can satisfy the ref, simply
log a warning and continue.

This enables a workflow where for network-isolated builds, the
subproject is cloned and moved into place and later on the build
orchestrator runs `meson subprojects update` without a network interface
to initialize the subproject.

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