Correct treatment of rexported modules in mkModuleNameProvidersMap

Before we would get the incorrect error message saying that the
rexporting package was the same as the defining package.

I think this only affects error messages for now.

```
-      it is bound as p-0.1.0.0:P2 by a reexport in package p-0.1.0.0
-      it is bound as P by a reexport in package p-0.1.0.0
+      it is bound as p-0.1.0.0:P2 by a reexport in package q-0.1.0.0
+      it is bound as P by a reexport in package r-0.1.0.0
```

and the output of `-ddump-mod-map` claimed..

```
Moo moo-0.0.0.1 (hidden package, reexport by moo-0.0.0.1)
```

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