IDE library manager leaves temporary directory name when libs are in a dropbox monitored path

When *File->Preferences->Sketchbook location* is set to a directory which is monitored by dropbox client, installation/upgrade of libraries interfere with dropbox client and do not get renamed to the proper final name of the lib but remain as temporary folder *arduino-\d+*.
Upon upgrade, this leads to multiple libraries in different directories and the builder not being able to consistently select the most recent version (see also issue https://github.com/arduino/arduino-builder/issues/366).
This issue is discussed on the forum thread https://forum.arduino.cc/index.php?topic=724169.

I tested with ArduinoIDE 1.8.13 on Windows 10 and a recent hourly build 2021-01-24 ... both showing the same behavior.

Attached a trace with Sysinternals Process Manager: [Logfile.zip](https://github.com/arduino/Arduino/files/5863472/Logfile.zip)
Filter settings: [ProcessMonitorFilterSettings](https://user-images.githubusercontent.com/4767099/105647649-67dcb900-5ea7-11eb-96db-b0a88ea3b9f9.jpg)
I suppose the interesting two lines are the ones with Result equal to SHARING VIOLATION and INVALID DEVICE REQUEST.

I also tested the IDE with directories monitored with the latest versions of nextcloud client and megasync client. Both worked well during fresh installs and up-/downgrades of libraries.
On the other hand, arduino-cli worked well with all three sync clients.

Reproduce on Windows with the Sketchbook location inside a Dropbox-monitored directory, then follow the IDE library installation and upgrade path while observing the temporary arduino-\d+ folder. Compare the behavior with arduino-cli and other sync clients; done when library folders are reliably renamed to their final names without duplicate versions.

Written by the indexing model from the issue text.