[Build] Use a separate scope while constructing a build plan (#7865)

### Motivation:

We need to use a separate inner scope to capture any problems during
build plan construction and make a decision to stop based on that. The
current logic checks the scope that is passed in which tracks errors
emitted elsewhere.

### Modifications:

- Passes an child scope called "Planning" to the `computeDestinations`
functions that is used to construct the plan.

### Result:

sourcekit-lsp no longer fails if there is no Package.resolved -
swiftlang/sourcekit-lsp#1618

Author: xedin

Sources/Build/BuildPlan/BuildPlan.swift

@@ -300,6 +300,7 @@ public class BuildPlan: SPMBuildCore.BuildPlan {
         var pluginDescriptions = [PluginBuildDescription]()
         var shouldGenerateTestObservation = true
 
+        let planningObservabilityScope = observabilityScope.makeChildScope(description: "Planning")
         try await Self.computeDestinations(
             graph: graph,
             onProduct: { product, destination in
@@ -317,7 +318,7 @@ public class BuildPlan: SPMBuildCore.BuildPlan {
                     toolsVersion: package.manifest.toolsVersion,
                     buildParameters: destination == .host ? toolsBuildParameters : destinationBuildParameters,
                     fileSystem: fileSystem,
-                    observabilityScope: observabilityScope
+                    observabilityScope: planningObservabilityScope
                 ))
             },
             onModule: { module, destination in
@@ -341,7 +342,7 @@ public class BuildPlan: SPMBuildCore.BuildPlan {
                                 product: product,
                                 forTarget: module,
                                 buildEnvironment: buildParameters.buildEnvironment,
-                                observabilityScope: observabilityScope
+                                observabilityScope: planningObservabilityScope
                                                         .makeChildScope(description: "Validate Deployment of Dependency")
                             )
                         }
@@ -358,7 +359,7 @@ public class BuildPlan: SPMBuildCore.BuildPlan {
                         additionalFileRules: additionalFileRules,
                         pkgConfigDirectories: pkgConfigDirectories,
                         fileSystem: fileSystem,
-                        observabilityScope: observabilityScope,
+                        observabilityScope: planningObservabilityScope,
                         surfaceDiagnostics: true
                     )
 
@@ -371,7 +372,7 @@ public class BuildPlan: SPMBuildCore.BuildPlan {
                         using: pluginConfiguration,
                         for: pluginInvocationResults,
                         fileSystem: fileSystem,
-                        observabilityScope: observabilityScope
+                        observabilityScope: planningObservabilityScope
                     )
                 }
 
@@ -396,7 +397,7 @@ public class BuildPlan: SPMBuildCore.BuildPlan {
                             shouldGenerateTestObservation: generateTestObservation,
                             shouldDisableSandbox: disableSandbox,
                             fileSystem: fileSystem,
-                            observabilityScope: observabilityScope
+                            observabilityScope: planningObservabilityScope
                         )
                     )
                 case is ClangModule:
@@ -410,7 +411,7 @@ public class BuildPlan: SPMBuildCore.BuildPlan {
                             buildToolPluginInvocationResults: buildToolPluginInvocationResults[module.id] ?? [],
                             prebuildCommandResults: prebuildCommandResults[module.id] ?? [],
                             fileSystem: fileSystem,
-                            observabilityScope: observabilityScope
+                            observabilityScope: planningObservabilityScope
                         )
                     )
                 case is PluginModule:
@@ -434,7 +435,7 @@ public class BuildPlan: SPMBuildCore.BuildPlan {
                             toolsVersion: package.manifest.toolsVersion,
                             buildParameters: toolsBuildParameters,
                             fileSystem: fileSystem,
-                            observabilityScope: observabilityScope
+                            observabilityScope: planningObservabilityScope
                         ))
                     }
 
@@ -459,7 +460,7 @@ public class BuildPlan: SPMBuildCore.BuildPlan {
         }
 
         // Abort now if we have any diagnostics at this point.
-        guard !observabilityScope.errorsReported else {
+        guard !planningObservabilityScope.errorsReported else {
             throw Diagnostics.fatalError
         }