[java-psi] Simplify PsiPackageImpl.isValid

It's probably ok to consider a package as valid, even if it doesn't contain any directories, and the prefix does not exist. Usually, invalid PsiElement is something deleted, or replaced, which cannot be used anymore. This is likely not the case here.
May help IDEA-368981 Renaming package never finishes, stuck "Looking for Usages"


(cherry picked from commit 27a08684daed8fd9232dbab3815e19deddd79b0d)

IJ-CR-160042

GitOrigin-RevId: 4a6291490f1687eaf459e3197bf0902760d0174b

java/java-psi-impl/src/com/intellij/psi/impl/file/PsiPackageImpl.java

@@ -106,9 +106,7 @@ public class PsiPackageImpl extends PsiPackageBase implements PsiPackage, Querya
 
   @Override
   public boolean isValid() {
-    return !getProject().isDisposed() &&
-           (PsiPackageImplementationHelper.getInstance().packagePrefixExists(this) ||
-            getDirectories().length > 0);
+    return !getProject().isDisposed();
   }
 
   @Override