Handle conflicts when moving files outside unity (diagnostics API)

[eelstork]

Here's a case:

• Create a new dir, file and import in Unity.
• Go outside unity, move the file outside the directory.
• Go back to unity and check what happens.
• Now delete the offending directory in Unity.
• Observe we now have an orphan in ~build

May demote this one specific use case but, probably need an option to automatically delete orphans which have a well formed counterpart somewhere else.

[eelstork]

Note: orphaned C# scripts should be automatically deleted (along with every C# script) on rebuild