chore: import upstream snapshot with attribution

Sources/ContainerPlugin/FilePath+Resolve.swift

@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2026 Apple Inc. and the container project authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+import SystemPackage
+
+extension FilePath {
+    /// Resolves a pathname string relative to this path.
+    ///
+    /// The result is lexically normalized — `.` components are removed and `..` components
+    /// collapse the preceding component. Absolute pathnames are returned normalized as-is;
+    /// relative pathnames are appended to `self` before normalizing.
+    ///
+    /// - Parameter pathname: The pathname to resolve.
+    /// - Returns: The resolved ``FilePath``, or `nil` if `pathname` is `nil` or empty.
+    package func resolve(_ pathname: String?) -> FilePath? {
+        guard let pathname, !pathname.isEmpty else { return nil }
+        let path = FilePath(pathname)
+        guard !path.isAbsolute else { return path.lexicallyNormalized() }
+        return self.appending(path.components).lexicallyNormalized()
+    }
+
+    /// Resolves a pathname string relative to this path, falling back to a default.
+    ///
+    /// The result is lexically normalized — `.` components are removed and `..` components
+    /// collapse the preceding component. Absolute pathnames are returned normalized as-is;
+    /// relative pathnames are appended to `self` before normalizing.
+    ///
+    /// - Parameters:
+    ///   - pathname: The pathname to resolve.
+    ///   - defaultPath: The path returned when `pathname` is `nil` or empty.
+    /// - Returns: The resolved ``FilePath``, or `defaultPath` lexically normalized if `pathname` is `nil` or empty.
+    package func resolve(_ pathname: String?, defaultPath: FilePath) -> FilePath {
+        resolve(pathname) ?? defaultPath.lexicallyNormalized()
+    }
+}