[FIR generator] Move Importable interface to common module

This is a step towards commonizing the code generator between
FIR and IR: KT-61970

compiler/ir/ir.tree/tree-generator/build.gradle.kts

@@ -10,6 +10,7 @@ runtimeOnly.extendsFrom(compileOnly)
 
 dependencies {
     implementation(project(":generators"))
+    implementation(project(":generators:tree-generator-common"))
     implementation(project(":core:compiler.common"))
     implementation(project(":compiler:util"))
     implementation("com.squareup:kotlinpoet:1.11.0")