patch 9.1.0369: Vim9: problem when importing autoloaded scripts

patch 9.1.0369: Vim9: problem when importing autoloaded scripts

Problem:  Vim9: problem when importing autoloaded scripts
Solution: In `:def` handle storing to vim9 autoload export
          (Ernie Rael)

Problem occurs when `import autoload ./.../autoload/...`. The autoload
in the specified path causes the use of an autoload_prefix which combines
with the `import autoload` to create trouble.

In `generate_store_var()` `case dest_script` use ISN_STOREEXPORT,
when needed, instead of ISN_STORES. When executing ISN_STOREEXPORT,
check for autoload_prefix.

fixes: #14606
closes: #14615

Signed-off-by: Ernie Rael <errael@raelity.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>