Move the usage of plugins from the dependencies parameter of targets in the package manifest to instead be in a separate plugins parameter.

Despite their similarity to dependencies we want users to consider them differently.  This also leaves room to grow in the future, when plugin usage might support parameters, etc.  We encode them separately in the manifest JSON, but merge them into regular dependencies to avoid duplicated implementation on the SwiftPM side.  This can be changed later without affecting clients.  For now it would just mean duplicate code.

rdar://74663667

Author: abertelrud

Sources/Build/BuildOperation.swift

@@ -192,19 +192,24 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
         }
         return outputPaths
     }
+    
     /// Create the build plan and return the build description.
     private func plan() throws -> BuildDescription {
+        // Load the package graph.
         let graph = try getPackageGraph()
+        
+        // Invoke any plugins in the graph, and get the results.
         let pluginInvocationResults = try getPluginInvocationResults(for: graph)
 
-        // Run any prebuild commands.  We get here in the case where there was no build description or it needs to change.
+        // Run any prebuild commands provided by plugins. Any failure stops the build.
         var prebuildOutputPaths: [ResolvedTarget: [AbsolutePath]] = [:]
         for target in graph.reachableTargets {
             if let results = pluginInvocationResults[target] {
                 prebuildOutputPaths[target] = try runPrebuildCommands(for: results)
             }
         }
-
+        
+        // Create the build plan based, on the graph and any information from plugins.
         let plan = try BuildPlan(
             buildParameters: buildParameters,
             graph: graph,
@@ -213,7 +218,8 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
             diagnostics: diagnostics
         )
         self.buildPlan = plan
-
+        
+        // Finally create the llbuild manifest from the plan.
         return try BuildDescription.create(with: plan)
     }