GhcPlugins: Fix lookup of TH names
Previously `thNameToGhcName` was calling `lookupOrigNameCache` directly, which failed to handle the case that the name wasn't already in the name cache. This happens, for instance, when the name was in scope in a plugin being used during compilation but not in scope in the module being compiled. In this case we the interface file containing the name won't be loaded and `lookupOrigNameCache` fails. This was the cause of #16104. The solution is simple: use the nicely packaged `lookupOrigIO` instead.
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