Add support for a "watchdog" based autoreloader

[Tom Forbes]

It was suggested on the mailing list (​https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/django-developers/Y3caNikdM6M/BYpsr0qLBAAJ) that an alternative reloader might be useful on Linux based systems.

I propose adding support for Watchdog (​https://github.com/gorakhargosh/watchdog) which will give us improved reloading performance on Linux and potentially Windows as well.

[Tom Forbes]

Potential patch: ​https://github.com/django/django/pull/11480

[Carlton Gibson]

Adam recently put together a proof-of-concept for a Watchfiles based reloader. See ​https://github.com/adamchainz/django-watchfiles

(​All these reloaders... it's quite complex 🤯).

Watchfiles is based on the Rust Notify crate, which had good cross-platform support. Arguably it would be the way to go.

Given how many options there are in this field, I wonder if we'd not be better served making get_reloader() properly pluggable, rather than adding (an ever growing set of) specific reloaders? 🤔

[Carlton Gibson <carlton@…>]

In e34dfad:

Refs #30213 -- Removed post-startup check for Watchman availability.

This is checked at startup in get_reloader(). The runtime check ties
the implementation to Watchman excessively.

[Ahmed Ibrahim]

We have a ticket (I'm the assignee) for using watchman, afaik, this one is no longer relevant