In commit v3.11.0-rc1~433^2~1 (Add support for IMPORTED GLOBAL targets
to be aliased, 2017-09-14) we accidentally dropped the error on calling
`add_library` to alias an imported target that is not globally visible.
The `add_executable` command's equivalent check was properly updated.
Restore the check in `add_library` with the same update.  Also fix the
test case accordingly.

Fixes: #17982