Support common metadata in kotlinx-metadata

KotlinCommonMetadata can be used to inspect .kotlin_builtins and
.kotlin_metadata files. It's in an internal package for now, but might
be published once the API is finalized.

core/metadata/src/org/jetbrains/kotlin/metadata/builtins/readPackageFragment.kt

@@ -0,0 +1,22 @@
+/*
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.metadata.builtins
+
+import org.jetbrains.kotlin.metadata.ProtoBuf
+import org.jetbrains.kotlin.protobuf.ExtensionRegistryLite
+import java.io.InputStream
+
+fun InputStream.readBuiltinsPackageFragment(): Pair<ProtoBuf.PackageFragment?, BuiltInsBinaryVersion> =
+    use { stream ->
+        val version = BuiltInsBinaryVersion.readFrom(stream)
+        val proto =
+            if (version.isCompatible()) ProtoBuf.PackageFragment.parseFrom(
+                stream,
+                ExtensionRegistryLite.newInstance().apply(BuiltInsProtoBuf::registerAllExtensions)
+            )
+            else null
+        proto to version
+    }