cacheLib() was unconditionally calling Load() for new caches even when
buffering was enabled, introduced by e013691d02 to fix issue #22191.
This broke "Save Library As" because the target file doesn't exist yet,
causing Load() to throw IO_ERROR. The exception was caught but no
symbols were added to the cache, resulting in an empty file
Additionally, the unbuffered library save path was passing the library
nickname instead of the target file path to the plugin's LoadSymbol/
SaveSymbol calls. While this was always wrong, it was masked before
e013691d02 because buffering mode skipped Load() entirely.
Fix cacheLib() to only call Load() for new caches when the library file
actually exists on disk, and fix the unbuffered save path to use the
target file path.
Fixes https://gitlab.com/kicad/code/kicad/-/issues/23337
Seth Hillbrand
fb1cab3e07