[FIR generator] Move Importable interface to common module

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

generators/tree-generator-common/build.gradle.kts

@@ -0,0 +1,15 @@
+plugins {
+    kotlin("jvm")
+    id("jps-compatible")
+}
+
+dependencies {
+    implementation(project(":core:compiler.common"))
+}
+
+sourceSets {
+    "main" {
+        projectDefault()
+    }
+    "test" {}
+}