Infer identifier namespace in elisp xref backend

Infer identifier namespace in elisp xref backend

Improve the accuracy of `xref-find-definitions` by inferring the
likely namespace of the sought identifier from its context.
This reduces the number of irrelevant search hits when it is clear
what kind of identifier is being looked for (such as showing a
variable when the user looks for a function).

Co-written with Dmitry Gutov.

* lisp/progmodes/elisp-mode.el (elisp--xref-list-index)
(elisp--xref-infer-namespace, xref-backend-identifier-at-point): New.
(xref-backend-definitions): Use the buffer position for inferring.
(elisp--xref-find-definitions): Use the inferred namespace.
(xref-backend-apropos): Adapt call.
* test/lisp/progmodes/elisp-mode-tests.el (elisp-mode-test--with-buffer)
(elisp-mode-with-buffer, elisp-mode-infer-namespace): New tests.