Search scope is prepared in UI thread and may cause UI freeze

In projects with long classpath or workspaces with lots of projects the calculation of the search scope can be expensive. Maybe it should be moved to the search job as well?

The UI freezes in my workspace on a M1 Mac are typically 18-20s.

From the log:

```
Stack Trace
at org.eclipse.jdt.internal.core.search.JavaSearchScope.addEnclosingProjectOrJar(JavaSearchScope.java:88)
at org.eclipse.jdt.internal.core.search.JavaSearchScope.add(JavaSearchScope.java:191)
at org.eclipse.jdt.internal.core.search.JavaSearchScope.add(JavaSearchScope.java:105)
at org.eclipse.jdt.internal.core.search.BasicSearchEngine.createJavaSearchScope(BasicSearchEngine.java:169)
at org.eclipse.jdt.internal.core.search.BasicSearchEngine.createJavaSearchScope(BasicSearchEngine.java:150)
at org.eclipse.jdt.core.search.SearchEngine.createJavaSearchScope(SearchEngine.java:357)
at org.eclipse.jdt.internal.ui.search.JavaSearchScopeFactory.createWorkspaceScope(JavaSearchScopeFactory.java:442)
at org.eclipse.jdt.internal.ui.search.JavaSearchScopeFactory.createWorkspaceScope(JavaSearchScopeFactory.java:435)
at org.eclipse.jdt.ui.actions.FindReferencesAction.createQuery(FindReferencesAction.java:103)
at org.eclipse.jdt.ui.actions.FindAction.run(FindAction.java:314)
at org.eclipse.jdt.ui.actions.FindAction.run(FindAction.java:297)
at org.eclipse.jdt.ui.actions.FindReferencesAction.run(FindReferencesAction.java:111)
at org.eclipse.jdt.ui.actions.FindAction.run(FindAction.java:260)
at org.eclipse.jdt.ui.actions.SelectionDispatchAction.dispatchRun(SelectionDispatchAction.java:278)
at org.eclipse.jdt.ui.actions.SelectionDispatchAction.run(SelectionDispatchAction.java:252)
at org.eclipse.jface.action.Action.runWithEvent(Action.java:474)
at org.eclipse.jface.commands.ActionHandler.execute(ActionHandler.java:121)
at org.eclipse.ui.internal.handlers.E4HandlerProxy.execute(E4HandlerProxy.java:97)
at jdk.internal.reflect.GeneratedMethodAccessor134.invoke(Unknown Source)
at java.base@17.0.9/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base@17.0.9/java.lang.reflect.Method.invoke(Method.java:568)
at org.eclipse.e4.core.internal.di.MethodRequestor.execute(MethodRequestor.java:58)
at org.eclipse.e4.core.internal.di.InjectorImpl.invokeUsingClass(InjectorImpl.java:300)
at org.eclipse.e4.core.internal.di.InjectorImpl.invoke(InjectorImpl.java:234)
at org.eclipse.e4.core.contexts.ContextInjectionFactory.invoke(ContextInjectionFactory.java:173)
at org.eclipse.e4.core.commands.internal.HandlerServiceHandler.execute(HandlerServiceHandler.java:156)
at org.eclipse.core.commands.Command.executeWithChecks(Command.java:488)
at org.eclipse.core.commands.ParameterizedCommand.executeWithChecks(ParameterizedCommand.java:485)
at org.eclipse.e4.core.commands.internal.HandlerServiceImpl.executeHandler(HandlerServiceImpl.java:213)
at org.eclipse.e4.ui.bindings.keys.KeyBindingDispatcher.executeCommand(KeyBindingDispatcher.java:308)
at org.eclipse.e4.ui.bindings.keys.KeyBindingDispatcher.press(KeyBindingDispatcher.java:580)
at org.eclipse.e4.ui.bindings.keys.KeyBindingDispatcher.processKeyEvent(KeyBindingDispatcher.java:655)
at org.eclipse.e4.ui.bindings.keys.KeyBindingDispatcher.filterKeySequenceBindings(KeyBindingDispatcher.java:439)
at org.eclipse.e4.ui.bindings.keys.KeyBindingDispatcher$KeyDownFilter.handleEvent(KeyBindingDispatcher.java:96)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:89)
at org.eclipse.swt.widgets.Display.filterEvent(Display.java:1217)
at org.eclipse.swt.widgets.Display.sendEvent(Display.java:4641)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1524)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1547)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1532)
at org.eclipse.swt.widgets.Widget.sendKeyEvent(Widget.java:1561)
at org.eclipse.swt.widgets.Widget.sendKeyEvent(Widget.java:1557)
at org.eclipse.swt.widgets.Canvas.sendKeyEvent(Canvas.java:522)
at org.eclipse.swt.widgets.Control.doCommandBySelector(Control.java:1101)
at org.eclipse.swt.widgets.Display.windowProc(Display.java:6492)
at org.eclipse.swt.internal.cocoa.OS.objc_msgSend(Native Method)
at org.eclipse.swt.internal.cocoa.NSResponder.interpretKeyEvents(NSResponder.java:59)
at org.eclipse.swt.widgets.Composite.keyDown(Composite.java:607)
at org.eclipse.swt.widgets.Display.windowProc(Display.java:6324)
at org.eclipse.swt.internal.cocoa.OS.objc_msgSendSuper(Native Method)
at org.eclipse.swt.widgets.Widget.callSuper(Widget.java:236)
at org.eclipse.swt.widgets.Widget.windowSendEvent(Widget.java:2264)
at org.eclipse.swt.widgets.Shell.windowSendEvent(Shell.java:2511)
at org.eclipse.swt.widgets.Display.windowProc(Display.java:6444)
at org.eclipse.swt.internal.cocoa.OS.objc_msgSendSuper(Native Method)
at org.eclipse.swt.widgets.Display.applicationSendEvent(Display.java:5692)
at org.eclipse.swt.widgets.Display.applicationProc(Display.java:5831)
at org.eclipse.swt.internal.cocoa.OS.objc_msgSend(Native Method)
at org.eclipse.swt.internal.cocoa.NSApplication.sendEvent(NSApplication.java:117)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3986)
at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine$5.run(PartRenderingEngine.java:1155)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:342)
at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.run(PartRenderingEngine.java:1046)
at org.eclipse.e4.ui.internal.workbench.E4Workbench.createAndRunUI(E4Workbench.java:155)
at org.eclipse.ui.internal.Workbench.lambda$3(Workbench.java:645)
at org.eclipse.ui.internal.Workbench$$Lambda$199/0x0000000800293780.run(Unknown Source)
at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:342)
at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:552)
at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:171)
at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:152)
at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:203)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:136)
at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:104)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:402)
at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:255)
at java.base@17.0.9/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base@17.0.9/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
at java.base@17.0.9/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base@17.0.9/java.lang.reflect.Method.invoke(Method.java:568)
at app//org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:651)
at app//org.eclipse.equinox.launcher.Main.basicRun(Main.java:588)
at app//org.eclipse.equinox.launcher.Main.run(Main.java:1459)
```

Start with JavaSearchScopeFactory.createWorkspaceScope and FindReferencesAction.createQuery, following the stack trace into BasicSearchEngine.createJavaSearchScope. Reproduce Find References in a workspace with a large classpath, then verify that scope preparation no longer freezes the UI and that the search still completes correctly.

Written by the indexing model from the issue text.