[Unity plugin] Fix the missing .meta for copied library.

[ganyuanzhen]

Fix #1146

[yuvaltassa]

@Balint-H LGTY?

[ganyuanzhen]

Already add some minor changes to the prompts and change the vaiable name to filePath. @Balint-H

[erez-tom]

Thanks for this! Two alternatives to consider:

1. Call Refresh after copy.
2. Use Unity's FileUtil instead of System. If you have reasons to reject both, we can accept the PR.