Enabling DevHome to recognize when the same file is removed.

[dhoehna]

Summary of the pull request

A user can not remove the "A file with the same name already exists" error in the DevDrive window by removing the file if they want to clone to the same location. This prevents users from cloning a DevDrive even if the file is removed.

The PR lets users add a DevDrive after getting the "A file with the same name exists" error. This can be done in two steps

1. The user removes the offending file.
2. The user re-enters the path (Either by browsing, or typing into the text box).

This is done in two steps.

1. Make the location change validate the drive label.
2. When the location changes, and the "A File already exists..." error is displayed, and the new location is the same as the old one, change Location to empty, then let it change to the "new" location.

References and relevant issues

Detailed description of the pull request / Additional comments

[ObservableProperty] will not fire a changed event if the new value is the same as the old value. Added a change to string.empty to make the validation code path light up.

Validation steps performed

Cloned a repo into a new dev drive after triggering the "A file already exists..." error.

PR checklist

• Closes After trying to create a Dev Drive where DevDrive.vhdx exists, the error persists even if the file is removed #2419
• Tests added/passed
• Documentation updated

[jsidewhite]