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

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

tests/pragmas/mused3.nim

@@ -0,0 +1,76 @@
+#[
+ran from trunner
+]#
+
+
+
+
+
+
+# line 10
+when defined case1:
+  from mused3a import nil
+  from mused3b import nil
+  mused3a.fn1()
+
+when defined case2:
+  from mused3a as m1 import nil
+  m1.fn1()
+
+when defined case3:
+  from mused3a import fn1
+  fn1()
+
+when defined case4:
+  from mused3a as m1 import fn1
+  m1.fn1()
+
+when defined case5:
+  import mused3a as m1
+  fn1()
+
+when defined case6:
+  import mused3a except nonexistent
+  fn1()
+
+when defined case7:
+  import mused3a
+  mused3a.fn1()
+
+when defined case8:
+  # re-export test
+  import mused3a except nonexistent
+  gn1()
+
+when defined case9:
+  # re-export test
+  import mused3a
+  gn1()
+
+when defined case10:
+  #[
+  edge case which happens a lot in compiler code:
+  don't report UnusedImport for mused3b here even though it works without `import mused3b`,
+  because `a.b0.f0` is accessible from both mused3a and mused3b (fields are given implicit access)
+  ]#
+  import mused3a
+  import mused3b
+  var a: Bar
+  discard a.b0.f0
+
+when false:
+  when defined case11:
+    #[
+    xxx minor bug: this should give:
+    Warning: imported and not used: 'm2' [UnusedImport]
+    but doesn't, because currently implementation in `markOwnerModuleAsUsed`
+    only looks at `fn1`, not fully qualified call `m1.fn1()
+    ]#
+    from mused3a as m1 import nil
+    from mused3a as m2 import nil
+    m1.fn1()
+
+when defined case12:
+  import mused3a
+  import mused3a
+  fn1()