Minor refactorings related to PackagePartProvider and implementations

- Document what exactly should findPackageParts return
- Make EMPTY a named object instead of a val in the companion
- Do not use JvmPackagePartProvider in tests where Empty works fine
- Add a couple default values to arguments of setupResolverForProject

compiler/frontend/src/org/jetbrains/kotlin/analyzer/AnalyzerFacade.kt

@@ -17,6 +17,7 @@
 package org.jetbrains.kotlin.analyzer
 
 import com.intellij.psi.search.GlobalSearchScope
+import org.jetbrains.kotlin.builtins.DefaultBuiltIns
 import org.jetbrains.kotlin.builtins.KotlinBuiltIns
 import org.jetbrains.kotlin.container.ComponentProvider
 import org.jetbrains.kotlin.context.ModuleContext
@@ -31,6 +32,7 @@ import org.jetbrains.kotlin.descriptors.impl.ModuleDescriptorImpl
 import org.jetbrains.kotlin.name.FqName
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.psi.KtFile
+import org.jetbrains.kotlin.resolve.CompilerEnvironment
 import org.jetbrains.kotlin.resolve.TargetEnvironment
 import org.jetbrains.kotlin.resolve.TargetPlatform
 import org.jetbrains.kotlin.resolve.createModule
@@ -147,10 +149,10 @@ abstract class AnalyzerFacade<in P : PlatformAnalysisParameters> {
             modules: Collection<M>,
             modulesContent: (M) -> ModuleContent,
             platformParameters: P,
-            targetEnvironment: TargetEnvironment,
-            builtIns: KotlinBuiltIns,
+            targetEnvironment: TargetEnvironment = CompilerEnvironment,
+            builtIns: KotlinBuiltIns = DefaultBuiltIns.Instance,
             delegateResolver: ResolverForProject<M> = EmptyResolverForProject(),
-            packagePartProviderFactory: (M, ModuleContent) -> PackagePartProvider = { module, content -> PackagePartProvider.EMPTY },
+            packagePartProviderFactory: (M, ModuleContent) -> PackagePartProvider = { module, content -> PackagePartProvider.Empty },
             firstDependency: M? = null
     ): ResolverForProject<M> {
         val storageManager = projectContext.storageManager