[flang] Fix .mod file bug with IMPORT of USEd name

If a symbol (derived type, for example) was use-associated into a scope
and then imported into a nested interface block, we were not including
the correct IMPORT statement in the .mod file.

This fixes refines the test for when the IMPORT is needed.

Fixes flang-compiler/f18#657.

Original-commit: flang-compiler/f18@8383de47ecb1b6901e2dd6f290df28fe8a470908
Reviewed-on: https://github.com/flang-compiler/f18/pull/675
Tree-same-pre-rewrite: false