Standardize auth-source queries in ERC

Standardize auth-source queries in ERC

* lisp/erc/erc.el (erc-password): Deprecate variable only used by
`erc-select-read-args'.  Server passwords are primarily used as
surrogates for other forms of authentication.  Such use is common but
nonstandard and often discouraged in favor of the de facto standard,
SASL.  Folks in the habit of invoking `erc(-tls)' interactively should
be encouraged to use auth-source instead.
(erc-select-read-args): Before this change, `erc-select-read-args'
offered to use the value of a non-nil `erc-password' as the :password
argument for `erc' and `erc-tls', referring to it as the "default"
password.  And when `erc-prompt-for-password' was nil and
`erc-password' wasn't, the latter was passed along unconditionally.
This only further complicated an already confusing situation for new
users, who in most cases shouldn't be worried about sending a PASS
command at all.  Until SASL arrives, they should provide server
passwords manually or learn to use auth-source.
(erc-auth-source-server-function, erc-auth-source-join-function): New
user options for retrieving a password externally, ostensibly by
calling `auth-source-search'.
(erc--auth-source-determine-params-defaults): New helper for
`erc--auth-source-search' with potential for exporting publicly in the
future.  Favors :host and :port fields above others.  Prioritizes
network IDs over announced servers and dialed endpoints.
(erc--auth-source-determine-params-merge): Add new function for
merging contextual and default parameters.  This is another contender
for possible exporting.
(erc--auth-source-search): New function for consulting auth-source and
sorting the result as filtered and prioritized by the previously
mentioned helpers.
(erc-auth-source-search): New function to serve as default
value for auth-source query-function options.
(erc-server-join-channel): Use user option for consulting auth-source
facility.  Also accept nil for first argument (instead of server).
(erc-cmd-JOIN): Use above-mentioned facilities when joining new
channel.  Omit server when calling `erc-server-join-channel'.  Don't
filter target buffers twice.  Don't call `switch-to-buffer', which
would create phantom buffers with names like target/server that were
never used.  IOW, only switch to existing target buffers.
(erc--compute-server-password): Add new helper function for
determining password.
(erc-open, erc-determine-parameters): Move password figuring from the
first to the latter.

* lisp/erc/erc-services.el
(erc-auth-source-services-function): Add new option for consulting
auth-source in a NickServ context.
(erc-nickserv-get-password): Pass network-context ID, when looking up
password in `erc-nickserv-passwords' and when formatting prompt for
user input.
(erc-nickserv-passwords): Add comment to custom option definition type
tag.

* test/lisp/erc/erc-services-tests.el: Add new test file for above
changes.  For now, stash auth-source-related tests here until a
suitable home can be found.

* lisp/erc/erc-join.el (erc-autojoin--join): Don't pass session-like
entity from `erc-autojoin-channels-alist' match to
`erc-server-join-channel'.  Allow that function to decide for itself
which host to look up if necessary.

* test/lisp/erc/resources/base/auth-source/foonet.eld: New file.
* test/lisp/erc/resources/base/auth-source/nopass.eld: New file.
* test/lisp/erc/resources/erc-scenarios-common.el: New file.
* test/lisp/erc/resources/services/auth-source/libera.eld: New file.
* test/lisp/erc/erc-scenarios-auth-source.el: New file.
* test/lisp/erc/erc-scenarios-base-reuse-buffers.el: New file.
* test/lisp/erc/erc-scenarios-join-auth-source.el: New file.
* test/lisp/erc/resources/base/reuse-buffers/channel/barnet.eld: New file.
* test/lisp/erc/resources/base/reuse-buffers/channel/foonet.eld: New file.
* test/lisp/erc/resources/join/auth-source/foonet.eld: New file.
(Bug#48598)