Don't link plugin dependencies to products

This fixes two issues with plugins and their dependencies:
- we were traversing plugin products and including their targets as part of any product
- since we are including plugin targets as dependencies as well, we could traverse plugin targets in a similarly incorrect way

Author: neonichu

Sources/Build/BuildPlan.swift

@@ -702,19 +702,22 @@ public class BuildPlan: SPMBuildCore.BuildPlan {
             switch dependency {
             // Include all the dependencies of a target.
             case .target(let target, _):
+                if [.macro, .plugin].contains(target.type) {
+                    return []
+                }
                 return target.dependencies.filter { $0.satisfies(self.buildEnvironment) }
 
             // For a product dependency, we only include its content only if we
-            // need to statically link it or if it's a plugin.
+            // need to statically link it.
             case .product(let product, _):
                 guard dependency.satisfies(self.buildEnvironment) else {
                     return []
                 }
 
                 switch product.type {
-                case .library(.automatic), .library(.static), .plugin:
+                case .library(.automatic), .library(.static):
                     return product.targets.map { .target($0, conditions: []) }
-                case .library(.dynamic), .test, .executable, .snippet, .macro:
+                case .library(.dynamic), .test, .executable, .snippet, .macro, .plugin:
                     return []
                 }
             }