Change the error message from "export declaration can only be used
within a module purview" to "export declaration can only be used within
a module interface" to be technically accurate.
The previous message was misleading because export declarations are
actually within a module purview when used in module implementation
units, but they are only allowed in module interface units.
This addresses the issue pointed out in GitHub issue #149008 where
Bigcheese noted that the diagnostic wording was incorrect.
Fixes#149008
fdec9fd4f8