[FIR] Do not include transitive friend dependencies in symbol provider

When flattening a dependency FirSymbolProvider, make sure transitive
dependency FirSymbolProviders are not included. This requires checking
that nested symbol provider sessions match the composite symbol provider
session when they are both source sessions.

^KT-60614 Fixed

compiler/testData/diagnostics/tests/multimodule/friendModule.txt

@@ -35,3 +35,8 @@ package p {
 package
 
 public fun test(): kotlin.Unit
+
+// -- Module: <m3> --
+package
+
+public fun test3(): kotlin.Unit