followup #18362: make UnusedImport work robustly (#18366)

* warnDuplicateModuleImport => hintDuplicateModuleImport
* improve DuplicateModuleImport msg, add test

tests/pragmas/mused3a.nim

@@ -0,0 +1,41 @@
+when defined case1:
+  proc fn1*() = discard
+
+when defined case2:
+  proc fn1*() = discard
+
+when defined case3:
+  proc fn1*() = discard
+  proc fn2*() = discard
+
+when defined case4:
+  proc fn1*() = discard
+  proc fn2*() = discard
+
+when defined case5:
+  proc fn1*() = discard
+
+when defined case6:
+  proc fn1*() = discard
+
+when defined case7:
+  proc fn1*() = discard
+
+when defined case8:
+  import mused3b
+  export mused3b
+
+when defined case9:
+  import mused3b
+  export mused3b
+
+when defined case10:
+  import mused3b
+  type Bar* = object
+    b0*: Foo
+
+when defined case11:
+  proc fn1*() = discard
+
+when defined case12:
+  proc fn1*() = discard