Fix logic for determining prefer-dynamic for a dylib.
The logic for determining if a dylib should use `prefer-dynamic` used to be something like "do not use prefer-dynamic if it is the current package".
The current logic has a strange behavior where it works as intended if there is only one package in the workspace, but a workspace with multiple packages will always use `prefer-dynamic`.
Instead of using `current_opt`, which isn't a good concept to use in a workspace, I switched this to be "primary" (a package selected on the command-line).
**History**
*  9879a0a  Initial prefer-dynamic behavior.
* #3221 changed to the faulty logic (see comments at #3221 (comment)). I think there was some confusion there.
* #3478 fixed the logic for one of the changed conditions, but not the one for prefer-dynamic