fix linting local packages

Plugins/SwiftLintPlugin/PathExtension.swift

@@ -26,6 +26,39 @@ extension Path {
     static let cat = Path("/bin/cat")
     static let sh = Path("/bin/sh")
 
+    private static let swiftlintConfig = ".swiftlint.yml"
+
+    /// Scans the receiver, then all of its parents looking for a configuration file with the name ".swiftlint.yml".
+    ///
+    /// - returns: Path to the configuration file, or nil if one cannot be found.
+    func firstParentContainingConfigFile() -> Path? {
+        let proposedDirectory = sequence(
+            first: self,
+            next: { path in
+                guard path.stem.count > 1 else {
+                    // Check we're not at the root of this filesystem, as `removingLastComponent()`
+                    // will continually return the root from itself.
+                    return nil
+                }
+
+                return path.removingLastComponent()
+            }
+        ).first { path in
+            let potentialConfigurationFile = path.appending(subpath: Self.swiftlintConfig)
+            return potentialConfigurationFile.isAccessible()
+        }
+        return proposedDirectory
+    }
+
+    /// Safe way to check if the file is accessible from within the current process sandbox.
+    private func isAccessible() -> Bool {
+        let result = string.withCString { pointer in
+            access(pointer, R_OK)
+        }
+
+        return result == 0
+    }
+
     /// Get file modification date
     var modified: Date {
         get throws {

Plugins/SwiftLintPlugin/SwiftLintPlugin.swift

@@ -43,7 +43,7 @@ struct SwiftLintPlugin: BuildToolPlugin {
         return try createBuildCommands(
             target: target.name,
             inputFiles: inputFiles,
-            packageDirectory: context.package.directory,
+            packageDirectory: context.package.directory.firstParentContainingConfigFile() ?? context.package.directory,
             workingDirectory: context.pluginWorkDirectory,
             tool: context.tool(named:)
         )