Handle command name/alias collision properly

[daxian-dbw]

Summary of the new feature / enhancement

Command names and aliases may collide. Today, we don't handle it, so an exception will be thrown when collision happens. We should handle the name/alias collision gracefully.

AIShell/shell/AIShell.Kernel/Command/CommandRunner.cs

Lines 61 to 67 in b10735a

	// The 'command.Name' is included in the 'command.Aliases' collection.
	// TODO: need to think about how to handle command names/aliases collision.
	// We don't handle collision today -- it will throw when collision happens.
	foreach (string alias in command.Aliases)
	{
	_commands.Add(alias, command);
	}

AIShell/shell/AIShell.Kernel/Command/CommandRunner.cs

Lines 89 to 94 in b10735a

	// The 'command.Name' is included in the 'command.Aliases' collection.
	// TODO: need to update accordingly when we handle command names/aliases collision.
	foreach (string alias in command.Aliases)
	{
	_commands.Remove(alias);
	}

Proposed technical implementation details (optional)

We load commands from an agent when switching to the agent and unload its commands when switching away from it. So, at a given time, only commands from one agent will co-exist with the core commands. Therefore, we shouldn't allow a name/alias to overwrite an existing one and hence we should throw a good error when collision happens.