Add debug strings to frontend scopes-related structures

Such as:
- PackageFragmentProviders, and, in particular,
CompositePackageFragmentProviders
- JavaPackageFragments
- Scopes produced by those providers

The rationale is that a lot of frontend-facing bugs (like red code) are
easily recognizeable in resolution. But at that point you just see a
bunch of scopes, without meaningful toStrings, you don't know who has
produced them, and what's exactly wrong.

With this commit it should make debugging slightly easier: now at least
you'll be able to see that "this scope is a scope of package fragment
for foo.bar of module baz" and decide whether the declaration should or
should not have been resolved from such scope.

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

@@ -22,7 +22,8 @@ import java.util.*
 import org.jetbrains.kotlin.name.Name
 
 class CompositePackageFragmentProvider(// can be modified from outside
-    private val providers: List<PackageFragmentProvider>
+    private val providers: List<PackageFragmentProvider>,
+    private val debugName: String
 ) : PackageFragmentProviderOptimized {
 
     init {
@@ -55,4 +56,6 @@ class CompositePackageFragmentProvider(// can be modified from outside
         }
         return result
     }
+
+    override fun toString(): String = debugName
 }

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

@@ -101,9 +101,12 @@ class ModuleDescriptorImpl @JvmOverloads constructor(
                 "Dependency module ${dependency.id} was not initialized by the time contents of dependent module ${this.id} were queried"
             }
         }
-        CompositePackageFragmentProvider(dependenciesDescriptors.map {
-            it.packageFragmentProviderForModuleContent!!
-        })
+        CompositePackageFragmentProvider(
+            dependenciesDescriptors.map {
+                it.packageFragmentProviderForModuleContent!!
+            },
+            "CompositeProvider@ModuleDescriptor for $name"
+        )
     }
 
     private val isInitialized: Boolean

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

@@ -22,12 +22,16 @@ import org.jetbrains.kotlin.descriptors.PackageFragmentDescriptor
 import org.jetbrains.kotlin.descriptors.SourceElement
 import org.jetbrains.kotlin.descriptors.annotations.Annotations
 import org.jetbrains.kotlin.name.FqName
+import org.jetbrains.kotlin.resolve.descriptorUtil.module
 
 abstract class PackageFragmentDescriptorImpl(
         module: ModuleDescriptor,
         final override val fqName: FqName
 ) : DeclarationDescriptorNonRootImpl(module, Annotations.EMPTY, fqName.shortNameOrSpecial(), SourceElement.NO_SOURCE),
         PackageFragmentDescriptor {
+    // Not inlined in order to not capture ref on 'module'
+    private val debugString: String = "package $fqName of $module"
+
     override fun <R, D> accept(visitor: DeclarationDescriptorVisitor<R, D>, data: D): R =
             visitor.visitPackageFragmentDescriptor(this, data)
 
@@ -39,5 +43,5 @@ abstract class PackageFragmentDescriptorImpl(
         return SourceElement.NO_SOURCE
     }
 
-    override fun toString(): String = "package $fqName"
+    override fun toString(): String = debugString
 }

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

@@ -66,4 +66,6 @@ open class SubpackagesScope(private val moduleDescriptor: ModuleDescriptor, priv
         p.popIndent()
         p.println("}")
     }
+
+    override fun toString(): String = "subpackages of $fqName from $moduleDescriptor"
 }