Is there a centralized place I can look to get this installed package list?
No, there isn't a central list of packages. As I said in the Github issue you opened, each package that is part of RetroPie has it's own list of dependencies, which is automatically installed when that particular package is installed. The dependencies are declared in the module's depends function. Since the user can install or remove any package, the list of dependencies can change.
A base installation will install the 'core' and 'main' packages, you can see here, you could get the list of dependencies for the packages in those 2 sections and consider them a minimum list of dependencies. Some of them might already be installed on the system, so when removing RetroPie you might take that into account.
EDIT: the initial dependencies for the RetroPie-Setup script to work are listed in the Manual installation page.