[Plugins] Make sure that plugins work when referenced by tools and destination modules

Remove incorrect assumption that the outputs of the plugin are always
connected to a destination build of a particular module and add a build
triple into the path to separate the outputs of a plugin.

This comes into play when i.e. a test is built for both `tools` and
`destination`, we need two plugin invocations to handle that.

Author: xedin

Sources/PackageGraph/BuildTriple.swift

@@ -16,12 +16,12 @@ import class PackageModel.Product
 /// Triple for which code should be compiled for.
 /// > Note: We're not using "host" and "target" triple terminology in this enum, as that clashes with build
 /// > system "targets" and can lead to confusion in this context.
-public enum BuildTriple {
+public enum BuildTriple: String {
     /// Triple for which build tools are compiled (the host triple).
-    case tools
+    case tools = "tools"
 
     /// Triple of the destination platform for which end products are compiled (the target triple).
-    case destination
+    case destination = "destination"
 }
 
 extension Target {

Sources/SPMBuildCore/Plugins/PluginInvocation.swift

@@ -452,7 +452,7 @@ extension ModulesGraph {
                 let toolPaths = accessibleTools.values.map { $0.path }.sorted()
 
                 // Assign a plugin working directory based on the package, target, and plugin.
-                let pluginOutputDir = outputDir.appending(components: package.identity.description, target.name, pluginTarget.name)
+                let pluginOutputDir = outputDir.appending(components: package.identity.description, target.name, target.buildTriple.rawValue, pluginTarget.name)
 
                 // Determine the set of directories under which plugins are allowed to write. We always include just the output directory, and for now there is no possibility of opting into others.
                 let writableDirectories = [outputDir]
@@ -597,9 +597,7 @@ extension ModulesGraph {
             }
 
             // Associate the list of results with the target. The list will have one entry for each plugin used by the target.
-            var targetID = target.id
-            targetID.buildTriple = .destination
-            pluginResultsByTarget[targetID] = (target, buildToolPluginResults)
+            pluginResultsByTarget[target.id] = (target, buildToolPluginResults)
         }
         return pluginResultsByTarget
     }

Tests/FunctionalTests/PluginTests.swift

@@ -226,7 +226,7 @@ final class PluginTests: XCTestCase {
 
         try testWithTemporaryDirectory { tmpPath in
             let packageDir = tmpPath.appending(components: "MyPackage")
-            let pathOfGeneratedFile = packageDir.appending(components: [".build", "plugins", "outputs", "mypackage", "SomeTarget", "Plugin", "best.txt"])
+            let pathOfGeneratedFile = packageDir.appending(components: [".build", "plugins", "outputs", "mypackage", "SomeTarget", "destination", "Plugin", "best.txt"])
 
             try createPackageUnderTest(packageDir: packageDir, toolsVersion: .v5_9)
             let (_, stderr) = try executeSwiftBuild(packageDir, env: ["SWIFT_DRIVER_SWIFTSCAN_LIB" : "/this/is/a/bad/path"])