Make code more resolved around URIs, not file paths

[felixfbecker]

LSP uses URIs to identify files. The LS uses file paths almost exclusively. This makes it impossible to, for example, to identify files that are not saved yet in the editor. It is not guaranteed that a URI can always be translated to a file path.

The TS language service probably only works with paths, but that translation should happen as late as possible, not as early as possible. In practice this means methods should take URIs where possible and classes like InMemoryFileSystem must be able to save files by URI.