If you want to migrate the FLS the best way is to create one of more profiles, as needed, with the appropriate FLS and include these in your package. These are not directly copied, but can be used as 'profile templates'. When the package is installed you select which 'profile template' to apply to each user profile in the target org.
However, I wouldn't normally expect you to use packages from sandbox to production - Change Sets will also do this (if you include both the fields being managed and the profiles in your change set).
If you want to migrate the FLS the best way is to create one of more profiles, as needed, with the appropriate FLS and include these in your package. These are not directly copied, but can be used as 'profile templates'. When the package is installed you select which 'profile template' to apply to each user profile in the target org.
However, I wouldn't normally expect you to use packages from sandbox to production - Change Sets will also do this (if you include both the fields being managed and the profiles in your change set).