[Analysis API] optimize KotlinPackageProvider.getSubPackageFqNames

Previously, we queried heavy kotlin package provider two times which affected performance
Now it's being queries only a single time

Also, the commit introduces separation for KotlinPackageProvider between kotlin and platform-specific packages

^KTIJ-24640

core/compiler.common/src/org/jetbrains/kotlin/name/Name.java

@@ -74,6 +74,11 @@ public final class Name implements Comparable<Name> {
         return true;
     }
 
+    public static @Nullable Name identifierIfValid(@NotNull String name) {
+        if (!isValidIdentifier(name)) return null;
+        return identifier(name);
+    }
+
     @NotNull
     public static Name special(@NotNull String name) {
         if (!name.startsWith("<")) {