Be more conservative with the lsp identifier guess

Be more conservative with the lsp identifier guess

If the user is not requesting a prompt, opt for the safer approach
which is to get the location from textDocument/definition, not from
workspace/symbol.  Because of things like function overloading, the
latter is not always successful in finding exactly the definition of
the thing one is invoking M-. on.

This requires using an xref-internal symbol, which is kind of
unfortunate.

* eglot.el (xref-backend-identifier-at-point): Rework.

GitHub-reference: per https://github.com/joaotavora/eglot/issues/131
GitHub-reference: per https://github.com/joaotavora/eglot/issues/314