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()

compiler/tests/org/jetbrains/kotlin/serialization/js/KotlinJavascriptSerializerTest.kt

@@ -57,7 +57,7 @@ public class KotlinJavascriptSerializerTest : TestCaseWithTmpdir() {
         val module = deserialize(metaFile)
 
         RecursiveDescriptorComparator.validateAndCompareDescriptorWithFile(
-                module.getPackage(TEST_PACKAGE_FQNAME)!!,
+                module.getPackage(TEST_PACKAGE_FQNAME),
                 RecursiveDescriptorComparator.DONT_INCLUDE_METHODS_OF_OBJECT,
                 File(source.replace(".kt", ".txt"))
         )