Fix a category of confusing scoping related bugs in access policies

[msullivan]

The edgeql->IR compiler has a notion of "weak namespaces" in which
scope tree nodes are tagged with namespaces, and those namespaces will
be dropped from a path if it refers to a node bound above that node.

The pg side doesn't understand any of this, though, so we have a
_rewrite_weak_namespaces that rewrites all affected path ids to
reference their targets directly.

Unfortunately we neglect to call it on our type rewrite expressions.
Fix that.

Also refactor fini_expression to try to make this category of bug
less likely by having more things driven by a list of expressions to
process.

[aljazerzen]

Suggested change

	extra_exprs = []
	extra_exprs += [
	rw for rw in ctx.env.type_rewrites.values()
	if isinstance(rw, irast.Set)
	]
	extra_exprs += [
	p.sub_params.decoder_ir for p in ctx.env.query_parameters.values()
	if p.sub_params and p.sub_params.decoder_ir
	]
	all_exprs = [ir] + extra_exprs
	all_exprs = [ir] + [
	rw for rw in ctx.env.type_rewrites.values()
	if isinstance(rw, irast.Set)
	] + [
	p.sub_params.decoder_ir for p in ctx.env.query_parameters.values()
	if p.sub_params and p.sub_params.decoder_ir
	]

[msullivan]

We can't get rid of extra_exprs, since we need to use that. We could build extra_exprs in one go, but I kind of like having each case more clearly specified, since it to me emphasizes that more things will be added