bug#50733: 28.0.1; project-find-regexp can block Emacs for a long time

["Daniel Martín via Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

When trying to search in a big project with thousands of files using
`project-find-regexp`, Emacs can synchronously block for a long time
until the results are computed for display in an *xref* buffer.

In contrast, the rgrep command and friends create a `grep-mode` buffer
and asynchronously fill it with results as they are computed by the grep
tool.  Emacs is not blocked and the user can start exploring matches
right away.

Is there a plan to eventually replace `grep-mode` with `xref-mode` in
Emacs?  If so, I think that computing results asynchronously is the most
important feature we'll lose.  When using `xref-mode` with an index
(etags, for example), the blocking is not noticeable; but when using a
grep tool the delay can be annoying if the repository is huge.

Another idea could be to use `grep-mode` for `project-find-regexp`, just
like `grep`, `lgrep`, `rgrep` and so one, but there's probably a reason
why it's using `xref-mode`.

Thanks.