Load definitions of symbols from .kotlin_metadata files

Extract AbstractDeserializedPackageFragmentProvider out of
JvmBuiltInsPackageFragmentProvider and implement it a little bit differently in
MetadataPackageFragmentProvider. The main difference is in how the package
fragment scope is constructed: for built-ins, it's just a single scope that
loads everything from one protobuf message. For metadata, package scope can
consist of many files, some of which store information about classes and others
are similar to package parts on JVM, so a ChainedMemberScope instance is
created.

Introduce a bunch of interfaces/methods to deliver the needed behavior to the
'deserialization' module which is not JVM-specific and does not depend on the
compiler code: MetadataFinderFactory,
PackagePartProvider#findMetadataPackageParts, KotlinMetadataFinder#findMetadata.
Note that these declarations are currently only implemented in the compiler; no
metadata package parts/fragments will be found in IDE or reflection

core/descriptors.runtime/src/org/jetbrains/kotlin/load/kotlin/reflect/ReflectKotlinClassFinder.kt

@@ -36,6 +36,9 @@ class ReflectKotlinClassFinder(private val classLoader: ClassLoader) : KotlinCla
         return findKotlinClass(javaClass.fqName?.asString() ?: return null)
     }
 
+    // TODO
+    override fun findMetadata(classId: ClassId): InputStream? = null
+
     // TODO: load built-ins from classLoader
     override fun findBuiltInsData(packageFqName: FqName): InputStream? = null
 }

core/descriptors.runtime/src/org/jetbrains/kotlin/load/kotlin/reflect/RuntimePackagePartProvider.kt

@@ -40,4 +40,7 @@ class RuntimePackagePartProvider(private val classLoader: ClassLoader) : Package
     override fun findPackageParts(packageFqName: String): List<String> {
         return module2Mapping.values.mapNotNull { it.findPackageParts(packageFqName) }.flatMap { it.parts }.distinct()
     }
+
+    // TODO
+    override fun findMetadataPackageParts(packageFqName: String): List<String> = TODO()
 }