Cleanup mistake with textdocumentitem and textdocumentidentifier

Cleanup mistake with textdocumentitem and textdocumentidentifier

Also introduce eglot--path-to-uri

* eglot.el (eglot--path-to-uri): Rename from eglot--uri and rework.
(eglot--connect): Use it.
(eglot--current-buffer-TextDocumentIdentifier): New function.
(eglot--current-buffer-VersionedTextDocumentIdentifier)
(eglot--signal-textDocument/didChange)
(eglot--signal-textDocument/didClose)
(eglot--signal-textDocument/willSave)
(eglot--signal-textDocument/didSave): Use it.