Check for re-import at translation-unit level

Previously the code checked for a duplicate `#import` using a data structure attached to the compile request, but this would fail for nested imports.
It also wouldn't work for a combination of `#import` and `__import`.

This change makes it so that we instead track a set of already-imported modules in the semantic checking visitor, which is instantiated once per translation unit.
We also key this set on the actual module (AST) imported, rather than on path/name/whatever, so hopefully it will be robust to the same thing getting imported multiple ways.

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