[K/N][cinterop] Improve Imports interface

Extracting `isImported(headerId: HeaderId): Boolean` method allows
mocking this interface in tests

kotlin-native/Interop/StubGenerator/src/main/kotlin/org/jetbrains/kotlin/native/interop/gen/Imports.kt

@@ -21,6 +21,8 @@ import org.jetbrains.kotlin.native.interop.indexer.*
 
 interface Imports {
     fun getPackage(location: Location): String?
+
+    fun isImported(headerId: HeaderId): Boolean
 }
 
 
@@ -35,6 +37,9 @@ class ImportsImpl(internal val headerIdToPackage: Map<HeaderId, PackageInfo>) :
         return packageInfo.name
     }
 
+    override fun isImported(headerId: HeaderId) =
+            headerId in headerIdToPackage
+
     private val accessedLibraries = mutableSetOf<KonanLibrary>()
 
     val requiredLibraries: Set<KonanLibrary>
@@ -63,11 +68,11 @@ class HeaderInclusionPolicyImpl(
 }
 
 class HeaderExclusionPolicyImpl(
-        private val importsImpl: ImportsImpl
+        private val imports: Imports
 ) : HeaderExclusionPolicy {
 
     override fun excludeAll(headerId: HeaderId): Boolean {
-        return headerId in importsImpl.headerIdToPackage
+        return imports.isImported(headerId)
     }
 
 }

kotlin-native/Interop/StubGenerator/src/main/kotlin/org/jetbrains/kotlin/native/interop/gen/jvm/main.kt

@@ -508,7 +508,7 @@ internal fun buildNativeLibrary(
         tool: ToolConfig,
         def: DefFile,
         arguments: CInteropArguments,
-        imports: ImportsImpl
+        imports: Imports
 ): NativeLibrary {
     val additionalHeaders = (arguments.header).toTypedArray()
     val additionalCompilerOpts = (arguments.compilerOpts +