Do not propagate main annotations to exported methods

[Cherry-picked ef4888b]

Author: nicolasstucki

Diff for: compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -1254,7 +1254,9 @@ class Namer { typer: Typer =>
           forwarder.addAnnotations(sym.annotations.filterConserve { annot =>
             annot.symbol != defn.BodyAnnot
             && annot.symbol != defn.TailrecAnnot
+            && annot.symbol != defn.MainAnnot
             && !annot.symbol.derivesFrom(defn.MacroAnnotationClass)
+            && !annot.symbol.derivesFrom(defn.MainAnnotationClass)
           })
 
           if forwarder.isType then

Diff for: tests/pos/export-main.scala

@@ -0,0 +1,5 @@
+object Foo:
+  @main def baz: Int = 1
+
+object Bar:
+  export Foo.baz // export Foo.baz but not create an new main entry point