fixes a converter handling regression that caused private converters to leak into client modules; fixes #19213; [backport:1.6] (#19229)

compiler/importer.nim

@@ -185,11 +185,13 @@ template addUnnamedIt(c: PContext, fromMod: PSym; filter: untyped) {.dirty.} =
   for it in mitems c.graph.ifaces[fromMod.position].converters:
     if filter:
       loadPackedSym(c.graph, it)
-      addConverter(c, it)
+      if sfExported in it.sym.flags:
+        addConverter(c, it)
   for it in mitems c.graph.ifaces[fromMod.position].patterns:
     if filter:
       loadPackedSym(c.graph, it)
-      addPattern(c, it)
+      if sfExported in it.sym.flags:
+        addPattern(c, it)
   for it in mitems c.graph.ifaces[fromMod.position].pureEnums:
     if filter:
       loadPackedSym(c.graph, it)

tests/converter/mdontleak.nim

@@ -0,0 +1,3 @@
+
+converter toBool(x: uint32): bool = x != 0
+# Note: This convertes is not exported!

tests/converter/tdontleak.nim

@@ -0,0 +1,10 @@
+discard """
+  output: '''5'''
+joinable: false
+"""
+
+import mdontleak
+# bug #19213
+
+let a = 5'u32
+echo a