Update source build externals documentation references (dotnet#5273)

Author: MichaelSimons

Documentation/sourcebuild-in-repos/new-dependencies.md

@@ -97,7 +97,7 @@ dependency and the nuances it may have.
   issue](https://github.com/dotnet/source-build/issues/new/choose) to determine
   if this is a workable approach for your foreign repo.
 1. We build some external dependencies in the
-  [dotnet/source-build-externals](https://github.com/dotnet/source-build-externals)
+  [dotnet/source-build-reference-packages](https://github.com/dotnet/source-build-reference-packages/tree/main/src/externalPackages/src)
   repo.  Good targets for this generally have very few if any dependencies and
   very simple build processes. [Please log an
   issue](https://github.com/dotnet/source-build/issues/new/choose) to get the

Documentation/sourcebuild-in-repos/update-dependencies.md

@@ -30,7 +30,8 @@ version all over again.  Please [log an
 issue](https://github.com/dotnet/source-build/issues/new/choose) to discuss
 this.  You can check if the external package you want is already included in
 source-build in the
-[source-build-externals](https://github.com/dotnet/source-build-externals) repo.
+[dotnet/source-build-reference-packages](https://github.com/dotnet/source-build-reference-packages/tree/main/src/externalPackages/src)
+repo.
 
 ## Splitting, combining, and moving packages