IR: add module ir.tree.impl, move main IR implementation there

compiler/ir/backend.jvm/build.gradle.kts

@@ -10,6 +10,7 @@ dependencies {
     compile(project(":compiler:ir.psi2ir"))
     compile(project(":compiler:ir.backend.common"))
     compile(project(":compiler:ir.serialization.jvm"))
+    compileOnly(project(":compiler:ir.tree.impl"))
     compileOnly(intellijCoreDep()) { includeJars("intellij-core", "asm-all", "guava", rootProject = rootProject) }
     compileOnly(intellijDep()) { includeJars("trove4j", rootProject = rootProject) }
 }

compiler/ir/ir.tree.impl/build.gradle.kts

@@ -0,0 +1,14 @@
+plugins {
+    kotlin("jvm")
+    id("jps-compatible")
+}
+
+dependencies {
+    compile(project(":compiler:ir.tree"))
+    compileOnly(intellijCoreDep()) { includeJars("intellij-core") }
+}
+
+sourceSets {
+    "main" { projectDefault() }
+    "test" {}
+}