Warn of absent networks module in ERC

* doc/misc/erc.texi: Add linkable note in Modules chapter about some
modules being required.  Also tweak markup in auth-source section.
* etc/ERC-NEWS: Mention the special role of `networks'.
* lisp/erc/erc-backend.el (erc--server-post-connect-hook): Add
internal hook for core modules to perform post-network-process,
pre-protocol config validation even when they haven't been loaded.
(erc--register-connection): Run `erc--server-post-connect-hook'.
* lisp/erc/erc-networks.el (erc-networks--bouncer-targets,
erc-networks-on-MOTD-end): Fix comments and doc strings.  Also change
former from constant to internal variable in case adjustment needed
between releases.
(erc-networks--warn-on-connect): New function to warn about the
`networks' module being absent from `erc-modules'.  This could
probably run at any time up to and including when the logical IRC
connection is established, but doing so at the process/protocol
boundary seems ideal.
* lisp/erc/erc-sasl.el (erc--register-connection): Defer to base
method instead of calling `erc-login' explicitly.
* lisp/erc/erc.el (erc-generate-new-buffer-name): Don't reconcile
buffer names when networks module not in play.
(erc-format-target-and/or-network): Don't assume networks module
loaded.
* test/lisp/erc/erc-scenarios-base-unstable.el:
(erc-scenarios-networks-no-module): New test.
* test/lisp/erc/resources/networks/no-module/basic.eld: New test data
file.  (Bug#60331.)

1 files changed, 10 insertions, 1 deletions

diff --git a/etc/ERC-NEWS b/etc/ERC-NEWS
index 76439f1d068..b577047ebcb 100644
--- a/etc/ERC-NEWS
+++ b/etc/ERC-NEWS
@@ -39,6 +39,14 @@ anew.  The pre-5.4 "disabled" behavior has been restored and will
 remain accessible for the foreseeable future, warts and all (e.g.,
 with its often superfluous "/DIALED-HOST" suffixing always present).
 
+** The 'networks' module is now quasi-required.
+The 'networks' module is now all but required for everyday interactive
+use.  A default member of 'erc-modules' since ERC 5.3, 'networks' has
+grown increasingly integral to core client operations over the years.
+From now on, only the most essential operations will be officially
+supported in its absence, and users will see a warning upon
+entry-point invocation when it's not present.
+
 ** Tighter auth-source integration with bigger changes on the horizon.
 The days of hit-and-miss auth-source queries are hopefully behind us.
 With the overhaul of the services module temporarily shelved and the
@@ -111,7 +119,8 @@ and 'erc-backend'.
 
 The function 'erc-network' always returns non-nil in server and target
 buffers belonging to a successfully established IRC connection, even
-after that connection has been closed.
+after that connection has been closed.  (Also see the note in the
+section above about the 'networks' module basically being mandatory.)
 
 In 5.4, support for network symbols as keys was added for
 'erc-autojoin-channels-alist'.  This has been extended to include