Since #4376, for-loops would set the loop variable outside, so it
stays valid.
They did this by doing the equivalent of
```fish
set -l foo $foo
for foo in 1 2 3
```
And that first imaginary `set -l` would also fire a set-event.
Since there's no use for it and the variable isn't actually set, we
remove it.
Fixes#8384.
for-loops that were not inside a function could overwrite global
and universal variables with the loop variable. Avoid this by making
for-loop-variables local variables in their enclosing scope.
This means that if someone does:
set a global
for a in local; end
echo $a
The local $a will shadow the global one (but not be visible in child
scopes). Which is surprising, but less dangerous than the previous
behavior.
The detection whether the loop is running inside a function was failing
inside command substitutions. Remove this special handling of functions
alltogether, it's not needed anymore.
Fixes#6480
