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/testsWithStdLib/experimental/useExperimentalOnWholeModule.txt

@@ -16,9 +16,6 @@ package api {
 // -- Module: <usage> --
 package
 
-package api {
-}
-
 package usage {
     public fun use(): kotlin.Unit
 }