Use vhdl_ls to track dependencies

[rcgoodfellow]

We are currently building a dependency graph through manual parsing of VHDL files. I think we can get a dependency-graph-like-object from vhdl_ls. If that is true, we should just use that.

[HelloKayT]

The Project API is pretty close to what you'd want except it's pretty tied into a text editor/usage for editing source e.g. all the find_* functions want to take a cursor into a text file. I can't tell whether "hacking" a cursor into existence is more or less work/brittle than manually walking the ASTs ourselves and searching for what we want.

The main issue is that the dependency API stuff that we really want isn't exposed thru the public API (VHDL-LS/rust_hdl#233). There is an open discussion about exposing the AST/dependency tree. It seems from the last update post (literally from like a week after I wrote a bunch of the vw stuff haha) like the tool may be almost ready to go for intrepid users? I don't think we do anything particularly insane dependency/definition-wise (maybe some VHDL-2019 stuff could throw it), so it could be worth reaching out maybe for a branch or something