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.

libraries/kotlinx-metadata/jvm/src/kotlinx/metadata/jvm/impl/JvmMetadataExtensions.kt

@@ -298,7 +298,12 @@ internal class JvmMetadataExtensions : MetadataExtensions {
     override fun createPackageExtension(): KmPackageExtension = JvmPackageExtension()
 
     override fun createModuleFragmentExtensions(): KmModuleFragmentExtension =
-        error("metadata-jvm doesn't have any extensions for module fragment!")
+        object : KmModuleFragmentExtension {
+            override val type: KmExtensionType = KmExtensionType(KmModuleFragmentExtension::class)
+
+            override fun accept(visitor: KmModuleFragmentExtensionVisitor) {
+            }
+        }
 
     override fun createFunctionExtension(): KmFunctionExtension = JvmFunctionExtension()
 

libraries/kotlinx-metadata/src/kotlinx/metadata/internal/common/KotlinCommonMetadata.kt

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2010-2022 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 kotlinx.metadata.internal.common
+
+import kotlinx.metadata.KmModuleFragment
+import kotlinx.metadata.KmModuleFragmentVisitor
+import kotlinx.metadata.impl.accept
+import org.jetbrains.kotlin.metadata.ProtoBuf
+import org.jetbrains.kotlin.metadata.builtins.readBuiltinsPackageFragment
+import org.jetbrains.kotlin.metadata.deserialization.NameResolverImpl
+import java.io.ByteArrayInputStream
+
+class KotlinCommonMetadata private constructor(private val proto: ProtoBuf.PackageFragment) {
+    fun toKmModuleFragment(): KmModuleFragment =
+        KmModuleFragment().apply(this::accept)
+
+    class Writer : KmModuleFragmentVisitor() {
+        // TODO
+    }
+
+    fun accept(v: KmModuleFragmentVisitor) {
+        val strings = NameResolverImpl(proto.strings, proto.qualifiedNames)
+        if (proto.hasPackage()) {
+            v.visitPackage()?.let { proto.`package`.accept(it, strings) }
+        }
+        for (klass in proto.class_List) {
+            v.visitClass()?.let { klass.accept(it, strings) }
+        }
+        v.visitEnd()
+    }
+
+    companion object {
+        @JvmStatic
+        fun read(bytes: ByteArray): KotlinCommonMetadata? {
+            val (proto, _) = ByteArrayInputStream(bytes).readBuiltinsPackageFragment()
+            if (proto == null) return null
+
+            return KotlinCommonMetadata(proto)
+        }
+    }
+}