Extract new method from the publicly used interface for compatibility

Note that we can't just use ``@JvmDefault` annotation as the main change
 is in the `core` module which targets Java 6

 #KT-42259 Fixed

core/descriptors/src/org/jetbrains/kotlin/descriptors/PackageFragmentProvider.kt

@@ -20,27 +20,35 @@ import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.name.Name
 
 interface PackageFragmentProvider {
-    fun collectPackageFragments(fqName: FqName, packageFragments: MutableCollection<PackageFragmentDescriptor>) {
-        // default implementation is provided only for BWC
-        packageFragments.addAll(getPackageFragments(fqName))
-    }
-
     @Deprecated("for usages use #packageFragments(FqName) at final point, for impl use #collectPackageFragments(FqName, MutableCollection<PackageFragmentDescriptor>)")
     fun getPackageFragments(fqName: FqName): List<PackageFragmentDescriptor>
 
     fun getSubPackagesOf(fqName: FqName, nameFilter: (Name) -> Boolean): Collection<FqName>
 
     object Empty : PackageFragmentProvider {
-        override fun collectPackageFragments(fqName: FqName, packageFragments: MutableCollection<PackageFragmentDescriptor>) {}
-
         override fun getPackageFragments(fqName: FqName) = emptyList<PackageFragmentDescriptor>()
 
         override fun getSubPackagesOf(fqName: FqName, nameFilter: (Name) -> Boolean) = emptySet<FqName>()
     }
 }
 
+interface PackageFragmentProviderOptimized : PackageFragmentProvider {
+    fun collectPackageFragments(fqName: FqName, packageFragments: MutableCollection<PackageFragmentDescriptor>)
+}
+
 fun PackageFragmentProvider.packageFragments(fqName: FqName): List<PackageFragmentDescriptor> {
     val packageFragments = mutableListOf<PackageFragmentDescriptor>()
-    collectPackageFragments(fqName, packageFragments)
+    collectPackageFragmentsOptimizedIfPossible(fqName, packageFragments)
     return packageFragments
 }
+
+fun PackageFragmentProvider.collectPackageFragmentsOptimizedIfPossible(
+    fqName: FqName,
+    packageFragments: MutableCollection<PackageFragmentDescriptor>
+) {
+    when (this) {
+        is PackageFragmentProviderOptimized -> collectPackageFragments(fqName, packageFragments)
+        else -> packageFragments.addAll(getPackageFragments(fqName))
+    }
+}
+

core/descriptors/src/org/jetbrains/kotlin/descriptors/PackageFragmentProviderImpl.kt

@@ -21,7 +21,7 @@ import org.jetbrains.kotlin.name.Name
 
 class PackageFragmentProviderImpl(
     private val packageFragments: Collection<PackageFragmentDescriptor>
-) : PackageFragmentProvider {
+) : PackageFragmentProviderOptimized {
     override fun collectPackageFragments(fqName: FqName, packageFragments: MutableCollection<PackageFragmentDescriptor>) {
         this.packageFragments.filterTo(packageFragments) { it.fqName == fqName }
     }

core/descriptors/src/org/jetbrains/kotlin/descriptors/impl/CompositePackageFragmentProvider.kt

@@ -18,13 +18,15 @@ package org.jetbrains.kotlin.descriptors.impl
 
 import org.jetbrains.kotlin.descriptors.PackageFragmentDescriptor
 import org.jetbrains.kotlin.descriptors.PackageFragmentProvider
+import org.jetbrains.kotlin.descriptors.PackageFragmentProviderOptimized
+import org.jetbrains.kotlin.descriptors.collectPackageFragmentsOptimizedIfPossible
 import org.jetbrains.kotlin.name.FqName
 import java.util.*
 import org.jetbrains.kotlin.name.Name
 
 class CompositePackageFragmentProvider(// can be modified from outside
     private val providers: List<PackageFragmentProvider>
-) : PackageFragmentProvider {
+) : PackageFragmentProviderOptimized {
 
     init {
         assert(providers.size == providers.toSet().size) {
@@ -35,14 +37,14 @@ class CompositePackageFragmentProvider(// can be modified from outside
     override fun getPackageFragments(fqName: FqName): List<PackageFragmentDescriptor> {
         val result = ArrayList<PackageFragmentDescriptor>()
         for (provider in providers) {
-            provider.collectPackageFragments(fqName, result)
+            provider.collectPackageFragmentsOptimizedIfPossible(fqName, result)
         }
         return result.toList()
     }
 
     override fun collectPackageFragments(fqName: FqName, packageFragments: MutableCollection<PackageFragmentDescriptor>) {
         for (provider in providers) {
-            provider.collectPackageFragments(fqName, packageFragments)
+            provider.collectPackageFragmentsOptimizedIfPossible(fqName, packageFragments)
         }
     }