Eglot: fix bug#70408 yet another way

Eglot: fix bug#70408 yet another way

The previous fix based on comparing the Eglot-provided didOpen URI to
the server-provided textDocument/publishDiagnostics URI didn't quite
work because the URI differs slightly in escaping conventions on certain
platforms.

This elephant-size bug is easily reproducible on Windows with clangd,
where every file is basically diagnostic-free.

* lisp/progmodes/eglot.el (eglot-path-to-uri): Rework.
(eglot--TextDocumentIdentifier-cache):
Rename from eglot--TextDocumentIdentifier-uri.
(eglot-handle-notification textDocument/publishDiagnostics): Tweak.
(eglot--TextDocumentIdentifier): Rework.
(eglot--signal-textDocument/didOpen): Tweak.

(cherry picked from commit 3a4583baf679289857150ee8ecf20b61e59b9d37)