Move importsDumper to compiler plugin

plugins/imports-dumper/testData/simpleCase/a/A1.kt

@@ -0,0 +1,7 @@
+package a
+
+import c.JavaC
+
+class A1 {
+    val j: JavaC = JavaC()
+}

plugins/imports-dumper/testData/simpleCase/a/A2.kt

@@ -0,0 +1,5 @@
+package a
+
+class A2 {
+    val a1: A1? = null
+}