Check for conflicting asset paths instead of filenames

This fixes a bug in which asset specifications for different subfolders are marked as conflicting when their filenames were equal. That is, an error is reported to the user when trying to generate `foo/layer1.png` and `bar/layer1.png`. This pull request addresses the problem at component-registration time by checking for conflicts against existing paths instead of existing filenames.

Addresses Watson issue 3803716 filed by @itoda. Please review, @volfied or @joelrbrandt.