Make ModuleDescriptor#getPackage() return not null lazy object with lazy scope

Refactor: no need to create package view in order to obtain its subpackages
LazyPackageViewDescriptorImpl to replace PackageViewDescriptorImpl
This allows to avoid computations when package views are requested but their contents not necessarily queried
For example: DescriptorResolver.resolvePackageHeader()

idea/ide-common/src/org/jetbrains/kotlin/resolve/lazy/ElementResolver.kt

@@ -181,7 +181,6 @@ public abstract class ElementResolver protected constructor(
                 if (trace.getBindingContext()[BindingContext.REFERENCE_TARGET, jetElement] == null) {
                     val fqName = header.getFqName(jetElement)
                     val packageDescriptor = resolveSession.getModuleDescriptor().getPackage(fqName)
-                                            ?: error("Package descriptor should be present in session for $fqName")
                     trace.record(BindingContext.REFERENCE_TARGET, jetElement, packageDescriptor)
                 }
             }
@@ -410,14 +409,13 @@ public abstract class ElementResolver protected constructor(
 
             // Inside import
             if (expression.getParentOfType<JetImportDirective>(false) != null) {
-                val rootPackage = resolveSession.getModuleDescriptor().getPackage(FqName.ROOT)!!
+                val rootPackage = resolveSession.getModuleDescriptor().getPackage(FqName.ROOT)
 
                 if (parent is JetDotQualifiedExpression) {
                     val element = parent.getReceiverExpression()
                     val fqName = expression.getContainingJetFile().getPackageFqName()
 
                     val filePackage = resolveSession.getModuleDescriptor().getPackage(fqName)
-                                      ?: error("File package should be already resolved and be found")
 
                     val descriptors = if (element is JetDotQualifiedExpression) {
                         qualifiedExpressionResolver.lookupDescriptorsForQualifiedExpression(
@@ -439,7 +437,7 @@ public abstract class ElementResolver protected constructor(
             val packageDirective = expression.getParentOfType<JetPackageDirective>(false)
             if (packageDirective != null) {
                 val packageDescriptor = resolveSession.getModuleDescriptor().getPackage(packageDirective.getFqName(expression as JetSimpleNameExpression).parent())
-                return packageDescriptor?.memberScope
+                return packageDescriptor.memberScope
             }
         }