Do not output empty non-root packages in tests with txt

Mostly to avoid listing every internal package in
kotlin-stdlib/kotlin-reflect, as in
https://github.com/JetBrains/kotlin/blob/acd5b62148d797ffd24c4811d1d663f0c1ae7885/compiler/testData/diagnostics/testsWithUnsignedTypes/conversions/conversionOfSignedToUnsigned.txt#L144

compiler/testData/diagnostics/tests/imports/TopLevelClassVsPackage.txt

@@ -50,6 +50,3 @@ package
 public fun test(/*0*/ a_b: a.b): kotlin.Unit
 public fun test2(/*0*/ _a: a): kotlin.Unit
 public fun test3(/*0*/ _a: a): kotlin.Unit
-
-package a {
-}