Default ExecuTorch targets to ExecuTorch-wide Buck visibility (#8969)

IMO, Buck visibility is just inverse deps. We should trust that people have a good reason to add deps rather than attempt to police them and require double entry in both deps and visibility, especially since we seem to be committed to APIs by default in OSS anyway.

Specific motivation is that #8712 would otherwise have to ad-hoc slap ExecuTorch-wide visibility on a lot of targets, but I've held this view for a long time.

Differential Revision: D70647462