Use standalone code paths by default

When users update fish by replacing files, existing shells might
throw weird errors because internal functions in share/functions/
might have changed.

This is easy to remedy by restarting the shell,
but I guess it would be nice if old shells kept using old data.
This is somewhat at odds with lazy-loading.

But we can use the standalone mode.  Turn that on by default.

Additionally, this could simplify packaging.  Some packages incorrectly
put third party files into /usr/share/fish/completion/ when they ought
to use /usr/share/fish/vendor_completions.d/ for that.  That packaging
mistake can make file conflicts randomly appearing when either fish
or foo ships a completion for foo.  Once we actually stop installing
/usr/share/fish/completions, there will no longer be a conflict (for
better or worse, things will silently not work, unless packagers
notice that the directory doesn't actually exist anymore).

The only advantage of having /usr/share/fish/ on the file system is
discoverability. But getting the full source code is better anyway.

Note that we still install (redundant) $__fish_data_dir when using
CMake.  This is to not unnecessarily break both
1. already running (old) shells as users upgrade to 4.2
2. plugins (as mentioned in an earlier commit),

We can stop installing $__fish_data_dir once we expect 99% of users
to upgrade from at least 4.2.

If we end up reverting this, we should try to get rid of the embed-data
knob in another way, but I'm not sure how.

Closes #11921

To-do:
- maybe make it a feature flag so users can turn it off without
  recompiling with "set -Ua no-embed-data".

doc_src/language.rst

@@ -381,7 +381,6 @@ By default ``$fish_function_path`` contains the following:
 - A directory for users to keep their own functions, usually ``~/.config/fish/functions`` (controlled by the ``XDG_CONFIG_HOME`` environment variable).
 - A directory for functions for all users on the system, usually ``/etc/fish/functions`` (really ``$__fish_sysconfdir/functions``).
 - Directories for other software to put their own functions. These are in the directories under ``$__fish_user_data_dir`` (usually ``~/.local/share/fish``, controlled by the ``XDG_DATA_HOME`` environment variable) and in the ``XDG_DATA_DIRS`` environment variable, in a subdirectory called ``fish/vendor_functions.d``. The default value for ``XDG_DATA_DIRS`` is usually ``/usr/share/fish/vendor_functions.d`` and ``/usr/local/share/fish/vendor_functions.d``.
-- The functions shipped with fish, usually installed in ``/usr/share/fish/functions`` (really ``$__fish_data_dir/functions``).
 
 If you are unsure, your functions probably belong in ``~/.config/fish/functions``.