Tool for updating 3rd-party notices should be more resilient to changes in dependencies

[NikolaMilosavljevic]

Currently we have a list of repos that contribute TPNs and a commented out list of those that do not. These lists can change and the repo can onboard its own TPN file. Every such change would require modification of the tool, to uncomment the repo-specific entry.

runtime/eng/regenerate-third-party-notices.proj

Lines 15 to 33 in d9cfbb4

	<ItemGroup>
	<TpnRepo Include="dotnet/runtime" />
	<TpnRepo Include="dotnet/aspnetcore" />
	<TpnRepo Include="dotnet/installer" />
	<TpnRepo Include="dotnet/roslyn-analyzers" />
	<TpnRepo Include="dotnet/templating" />
	<TpnRepo Include="dotnet/winforms" />
	<TpnRepo Include="dotnet/wpf" />
	<!--
	Additional repos that should be included but don't have any third-party-notices files:
	dotnet/efcore
	dotnet/emsdk
	dotnet/extensions
	dotnet/icu
	dotnet/sdk
	dotnet/windowsdesktop
	dotnet/linker
	-->

Instead of requiring manual tool changes, which can easily be overlooked, make a change to the tool to check all known repos and skip those that do not have TPNs onboarded.

[NikolaMilosavljevic]

[Triage] We could use Darc dependencies, starting with installer, to discover all product repos.

[akoeplinger]

We also need to remove the dotnet/installer repo from this list now that it was merged into the sdk repo.
I tried adding the sdk repo instead but then the target complains because the TPN file in the sdk repo doesn't have any entries (which sounds like another issue?)