Add declarations from serialization plugin to FIR metadata declarations provider.

These declarations should not be visible to users (and therefore are not added to FIR),
but plugin itself can reference them in already compiled serializable classes,
and therefore they should be available in metadata:

- synthetic deserialization constructor
- static write$Self function

See also:
^KT-55885

plugins/kotlinx-serialization/kotlinx-serialization.backend/build.gradle.kts

@@ -12,6 +12,8 @@ dependencies {
     compileOnly(project(":compiler:backend.jvm.codegen"))
     compileOnly(project(":compiler:backend.jvm.lower"))
     compileOnly(project(":compiler:ir.tree"))
+    compileOnly(project(":compiler:fir:fir2ir"))
+    compileOnly(project(":compiler:fir:tree"))
     compileOnly(project(":js:js.frontend"))
     compileOnly(project(":js:js.translator"))
     compileOnly(project(":kotlin-util-klib-metadata"))