Do not include all of the build dir for API digester to avoid conflicting definitions. (#8776)

Do not include all of the build dir for API digester to avoid
conflicting definitions.

### Motivation:

Issue #8081 : when a package or its dependencies define a tool plugin,
then diagnose-api-breaking-changes report false API breakage.

Investigation shown that diagnose-api-breaking-changes passes for
analysis (with `-I`) all of the build dir, with all of the dependencies.
When a tool plugin is in the dependency tree, then dependencies for the
plugin itself are built with `.host` destination. If it happens that
these modules are also built for `.target` destination, then
`swift-api-digester` fails to analyze them, and public API declared
there is printed as removed.

```
/Users/yy/work/pub/tst/.build/arm64-apple-macosx/debug/MyLib-tool.build/module.modulemap:1:8: error: redefinition of module 'MyLib'
module MyLib {
       ^
/Users/yy/work/pub/tst/.build/arm64-apple-macosx/debug/MyLib.build/module.modulemap:1:8: note: previously defined here
module MyLib {
       ^
Failed to load module: MyLib
```

### Modifications:

The goal for this PR is to reduce analyzed surface from "all of the
build directory" down to modules built for target.

### Result:

Before the change, example in Issue #8081 

```
1 breaking change detected in mypackage:
  💔 API breakage: var myPublicString has been removed
```

After the change:

```
No breaking changes detected in mypackage
```

Author: yyvch

Fixtures/Miscellaneous/APIDiff/WithPlugin/Package.swift

@@ -0,0 +1,16 @@
+// swift-tools-version: 6.1
+import PackageDescription
+
+let package = Package(
+    name: "package-with-plugin",
+    products: [.library(name: "PackageLib", targets: ["TargetLib"])],
+    targets: [
+        .target(name: "TargetLib"),
+        .executableTarget(name: "BuildTool", dependencies: ["TargetLib"]),
+        .plugin(
+            name: "BuildPlugin",
+            capability: .command(intent: .custom(verb: "do-it-now", description: "")),
+            dependencies: ["BuildTool"]
+        ),
+    ]
+)

Fixtures/Miscellaneous/APIDiff/WithPlugin/Plugins/BuildPlugin/BuildToolPlugin.swift

@@ -0,0 +1,9 @@
+import Foundation
+import PackagePlugin
+
+@main
+final class BuildToolPlugin: CommandPlugin {
+    func performCommand(context: PluginContext, arguments: [String]) async throws {
+        _ = try context.tool(named: "BuildTool")
+    }
+}

Fixtures/Miscellaneous/APIDiff/WithPlugin/Sources/BuildTool/BuildTool.swift

@@ -0,0 +1,8 @@
+import TargetLib
+
+@main
+public struct BuildTool {
+    static func main() {
+        TargetLibStruct.do_it()
+    }
+}

Fixtures/Miscellaneous/APIDiff/WithPlugin/Sources/TargetLib/TargetLib.swift

@@ -0,0 +1,5 @@
+public enum TargetLibStruct {
+    public static func do_it() {
+        print("Hello!")
+    }
+}

Sources/Build/BuildPlan/BuildPlan.swift

@@ -585,8 +585,7 @@ public class BuildPlan: SPMBuildCore.BuildPlan {
 
     public func createAPIToolCommonArgs(includeLibrarySearchPaths: Bool) throws -> [String] {
         // API tool runs on products, hence using `self.productsBuildParameters`, not `self.toolsBuildParameters`
-        let buildPath = self.destinationBuildParameters.buildPath.pathString
-        var arguments = ["-I", buildPath]
+        var arguments: [String] = []
 
         // swift-symbolgraph-extract does not support parsing `-use-ld=lld` and
         // will silently error failing the operation.  Filter out this flag
@@ -610,7 +609,12 @@ public class BuildPlan: SPMBuildCore.BuildPlan {
         for target in self.targets {
             switch target {
             case .swift(let targetDescription):
-                arguments += ["-I", targetDescription.moduleOutputPath.parentDirectory.pathString]
+                if target.destination == .target {
+                    // Include in the analysis surface target destination. That way auxiliary
+                    // modules from building a build tool (destination == .host) won't conflict
+                    // with the modules intended to analyze.
+                    arguments += ["-I", targetDescription.moduleOutputPath.parentDirectory.pathString]
+                }
             case .clang(let targetDescription):
                 if let includeDir = targetDescription.moduleMap?.parentDirectory {
                     arguments += ["-I", includeDir.pathString]

Tests/CommandsTests/APIDiffTests.swift

@@ -328,6 +328,17 @@ class APIDiffTestCase: CommandsBuildProviderTestCase {
         }
     }
 
+
+    func testAPIDiffPackageWithPlugin() async throws {
+        try skipIfApiDigesterUnsupportedOrUnset()
+        try await fixture(name: "Miscellaneous/APIDiff/") { fixturePath in
+            let packageRoot = fixturePath.appending("WithPlugin")
+            let (output, _) = try await execute(["diagnose-api-breaking-changes", "1.2.3"], packagePath: packageRoot)
+            XCTAssertMatch(output, .contains("No breaking changes detected in TargetLib"))
+        }
+    }
+
+
     func testBadTreeish() async throws {
         try skipIfApiDigesterUnsupportedOrUnset()
         try await fixture(name: "Miscellaneous/APIDiff/") { fixturePath in