WIP Initial work on #3922

This is nowhere near ready, it just paints a basic idea of a direction
to start in. This is based on a whiteboard drawing I did, which may or
may not be helpful.

https://imgur.com/a/1jv3WLp

package.yaml

@@ -158,7 +158,6 @@ library:
   - Paths_stack
   - Stack.Build
   - Stack.Build.Cache
-  - Stack.Build.ConstructPlan
   - Stack.Build.Execute
   - Stack.Build.Haddock
   - Stack.Build.Installed
@@ -234,6 +233,7 @@ library:
   - Stack.Sig.Sign
   - Stack.Snapshot
   - Stack.Solver
+  - Stack.SourceMap
   - Stack.StaticBytes
   - Stack.Types.Build
   - Stack.Types.BuildPlan
@@ -256,6 +256,7 @@ library:
   - Stack.Types.Resolver
   - Stack.Types.Runner
   - Stack.Types.Sig
+  - Stack.Types.SourceMap
   - Stack.Types.TemplateName
   - Stack.Types.Version
   - Stack.Types.VersionIntervals