performance: avoid O(n^2) in PDEJavaHelper

findPackageFragmentRoot() searches through all PackageFragment Roots and
is called for every libPaths. This can be slow due to involved file
access.
see eclipse-jdt/eclipse.jdt.core#303

Instead call getAllPackageFragmentRoots() only once, index the result
and use O(1) hash access.

ui/org.eclipse.pde.core/src/org/eclipse/pde/internal/core/util/PDEJavaHelper.java

@@ -19,6 +19,7 @@
 import java.util.HashMap;
 import java.util.LinkedHashMap;
 import java.util.ListIterator;
+import java.util.Map;
 
 import org.eclipse.core.resources.IProject;
 import org.eclipse.core.resources.IResource;
@@ -286,9 +287,16 @@ private static IPackageFragment searchWorkspaceForPackage(String packageName, IP
 					continue;
 				}
 				IJavaProject jp = JavaCore.create(projects[i]);
+				Map<IPath, IPackageFragmentRoot> rootsByPath = new HashMap<>();
+				for (IPackageFragmentRoot classpathRoot : jp.getAllPackageFragmentRoots()) {
+					IPath classRootPath = classpathRoot.getPath();
+					if (classRootPath != null) {
+						rootsByPath.put(classRootPath, classpathRoot);
+					}
+				}
 				ListIterator<IPath> li = libPaths.listIterator();
 				while (li.hasNext()) {
-					IPackageFragmentRoot root = jp.findPackageFragmentRoot(li.next());
+					IPackageFragmentRoot root = rootsByPath.get(li.next());
 					if (root != null) {
 						IPackageFragment frag = root.getPackageFragment(packageName);
 						if (frag.exists()) {