Move generated completions to cache directory

[Accurate0]

Description

Generates completions in XDG_CACHE_HOME or ~/.cache by default and loads them from there.

Not sure what we want to about existing generated completions? This PR basically updates the paths and ignores the old ones, not sure if this change is considered breaking in that way...

Fixes #10369

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[krobelus]

The breakage in ninja -Cbuild test_expansion.fish is

Testing file checks/expansion.fish ... Failure:

  The CHECKERR on line 316 wants:
    fish: Expected a variable name after this $.

  which failed to match line stderr:22:
    rmdir: failed to remove '/tmp/tmp.fTr6tzPmvD/temp/tmp.tPB4txOGmK/realhome': Directory not empty

which means that we should teach our test framework to set XDG_CACHE_HOME similar to how it already sets XDG_CONFIG_HOME and others. This way the "realhome" will be empty

[krobelus]

tests/test_env.sh:44:1:XDG_CONFIG_HOME="$homedir/xdg_config_home"
tests/test_env.sh:45:8:export XDG_CONFIG_HOME
tests/test_env.sh:46:11:mkdir -p $XDG_CONFIG_HOME/fish || die

need to add it in this file

[Accurate0]

tests/test_env.sh:44:1:XDG_CONFIG_HOME="$homedir/xdg_config_home"
tests/test_env.sh:45:8:export XDG_CONFIG_HOME
tests/test_env.sh:46:11:mkdir -p $XDG_CONFIG_HOME/fish || die

need to add it in this file

ah great thank you, I was trying get my head around how the testing was setup, makes sense now.

is there any test that makes sense to add or amend? I didn't see anything existing that seemed to relate

[krobelus]

We don't have tests for this yet. Would be good but not absolutely necessary