Fixes #4061

[RedHatter]

Fixes #4061 where changes to let directives where not being passed to nested slots. I feel like I'm missing something though. Why was filtering let dependencies (non_let_dependencies) thought to be necessary in the first place?

In light of this fix I'm also suspicious of lines 94-103

const dependencies: Set<string> = new Set();

// TODO is this filtering necessary? (I *think* so)
default_slot.dependencies.forEach(name => {
	if (!this.node.scope.is_let(name)) {
		dependencies.add(name);
	}
});

block.add_dependencies(dependencies);

It looks to be doing something similar but I'm not sure what its exact effect is. The issue is solved without removing line 98 but I wonder if it's causing some other subtle bug.