lsp: Refactor read files tracking

[flofriday]

Right now the LS builds a dependency map and get's the read files from a custom VirtualFileSystem. This can now be much simpler as since #893 the AST has the method List<Path> allReadFiles() or even more granularly List<Path> includedFiles(boolean recursive).

[jaraonthe-dot-net]

Note to self:

• Still need the dependency map as we need to know which files a specific file has been included by.
• Verify: Is there ever a situation where we don't receive an AST from the parser (or it doesn't provide all included files because it is incomplete) but really need the information for the dependency map? (The current solution picks up on each file the Parser opened, not just included in the AST.)