Only rescan libraries folders when really needed

Scanning libraries is an heavy task if the sketchbook becomes huge;
This patch targets two points:

- remove the rescan() after setLibrariesFolders(), which already performs a rescan
- call setLibrariesFolders() only when the folder list has changed
  - This ensures that no scan is performed when changing board in the same architecture

Could mitigate #6350

arduino-core/src/cc/arduino/contributions/libraries/LibrariesIndexer.java

@@ -106,6 +106,10 @@ public class LibrariesIndexer {
     rescanLibraries();
   }
 
+  public List<File> getLibrariesFolders() {
+    return librariesFolders;
+  }
+
   public void rescanLibraries() {
     // Clear all installed flags
     installedLibraries.clear();

arduino-core/src/processing/app/BaseNoGui.java

@@ -679,8 +679,9 @@ public class BaseNoGui {
     // Libraries located in the latest folders on the list can override
     // other libraries with the same name.
     librariesIndexer.setSketchbookLibrariesFolder(getSketchbookLibrariesFolder());
-    librariesIndexer.setLibrariesFolders(librariesFolders);
-    librariesIndexer.rescanLibraries();
+    if (librariesIndexer.getLibrariesFolders() == null || !librariesIndexer.getLibrariesFolders().equals(librariesFolders)) {
+      librariesIndexer.setLibrariesFolders(librariesFolders);
+    }
 
     populateImportToLibraryTable();
   }