Flattening issue when importing variables by different paths with different names

[bugarela]

The example on this PR results in a invalid flattened module https://github.com/informalsystems/quint/pull/1363.

I tried fixing this in the compiler, which is enough for the simulator. But Apalache can't handle multiple names for the same variable. The flattener should ensure we always refer to that variable with the same name.