Introduce LocalOrigin + PathOrigin & schema.PathTarget

[radeksimko]

This PR introduces a concept of a path-based origin, i.e. origin reference that is pointing to a different path, in Terraform that would be outside of a module.

In Terraform this can be represented by a module input name, pointing to the relevant variable, e.g.

module "example" {
  source = "./module"
  foo = "bar"
}

where foo would be the PathOrigin and point to this block within ./module/

variable "foo" {
  type = string
}

Relatedly all the existing origins become LocalOrigin with all the same existing structure.

There were a few high-level API changes in that both reference-lookup-related methods were moved under Decoder (from PathDecoder) as they can now both return results outside the context of a single path and therefore also need access to other paths via the PathReader.

It's not the smallest PR, but also it's not as big as some other recent ones 😅 and hopefully the commit log can be helpful in walking through it.

[dbanck]

I was able to follow the changes in each commit and think everything makes sense so far. However, I cannot provide a "real" approval (yet) because everything is still new, and I can't oversee all implications.