Rename rpc methods for clarity

Rename rpc methods for clarity

* eglot.el (eglot--process-receive): Search for RPC server methods
under `eglot--server-'
(eglot-editing-mode, eglot--maybe-activate-editing-mode): Use new
signal names.
(eglot--server-window/showMessage): Rename from
eglot--window/showMessage.
(eglot--server-textDocument/publishDiagnostics): Renamed from
eglot--textDocument/publishDiagnostics.
(eglot--current-buffer-versioned-identifier): Remove.
(eglot--current-buffer-VersionedTextDocumentIdentifier): Use
eglot--versioned-identifier.
(eglot--signal-textDocument/didChange): Renamed from
eglot--maybe-signal-didChange.
(eglot--signal-textDocument/didOpen): Renamed from
eglot--signalDidOpen.
(eglot--signal-textDocument/didClose): Rename from
eglot--signalDidClose.
(eglot-flymake-backend): Call eglot--signal-textDocument/didChange.
(eglot--server-window/progress): Rename from
eglot--window/progress.