Duplicate File action only works on open files

[Nokel81]

Describe the bug
When executing the "Duplicate File" actions from the command pallet (or more generally a keyboard shortcut) and no file is currently open (but the explorer is active and an item is selected) an error occurs saying that a file needs to be open. This also means that if you have a file open but the explorer is what is currently active you cannot duplicate the currently active selection

To Reproduce
Steps to reproduce the behavior:

1. Close all files
2. Select a file in the explorer
3. Call up the command pallet and execute the file duplicate action
4. An error is displayed

Expected behavior
The currently selected file/folder is duplicated

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• VSCode Version: 1.39.0
• OS Version: 10.0.17763

[dmn-seasony]

A related issue is that duplicating a folder using a keybinding or the command palette doesn't work (it duplicates the currently open file instead). Both issues are caused by the fact that VSCode doesn't expose an API to get the currently selected file/folder from the explorer view. See microsoft/vscode#3553, where some possible workarounds are also mentioned.