Make the Runfiles compatible with Swift 6 language mode (#1617)

Additionally fixed indentation.
Changing `func repository(from path: String)` was not necessary, but
decided to swiftify.

[Should Regex be
Sendable?](https://forums.swift.org/t/should-regex-be-sendable/69529) on
swift forums.

---------

Co-authored-by: Aaron Sky <aaronsky@skyaaron.com>

Author: kirillyakimovich

swift/runfiles/Runfiles.swift

@@ -214,22 +214,21 @@ public final class Runfiles {
     }
 }
 
-  // https://github.com/bazel-contrib/rules_go/blob/6505cf2e4f0a768497b123a74363f47b711e1d02/go/runfiles/global.go#L53-L54
-  private let legacyExternalGeneratedFile = /bazel-out\/[^\/]+\/bin\/external\/([^\/]+)/
-  private let legacyExternalFile = /external\/([^\/]+)/
-
-  // Extracts the canonical name of the repository containing the file
-  // located at `path`.
-  private func repository(from path: String) -> String {
-    if let match = path.prefixMatch(of: legacyExternalGeneratedFile) {
-      return String(match.1)
-    }
-    if let match = path.prefixMatch(of: legacyExternalFile) {
-      return String(match.1)
-    }
-    // If a file is not in an external repository, return an empty string
-    return ""
-  }
+// https://github.com/bazel-contrib/rules_go/blob/6505cf2e4f0a768497b123a74363f47b711e1d02/go/runfiles/global.go#L53-L54
+#if swift(>=5.10)
+nonisolated(unsafe) private let legacyExternalGeneratedFile = /bazel-out\/[^\/]+\/bin\/external\/([^\/]+)/
+nonisolated(unsafe) private let legacyExternalFile = /external\/([^\/]+)/
+#else
+private let legacyExternalGeneratedFile = /bazel-out\/[^\/]+\/bin\/external\/([^\/]+)/
+private let legacyExternalFile = /external\/([^\/]+)/
+#endif
+
+// Extracts the canonical name of the repository containing the file
+// located at `path`.
+private func repository(from path: String) -> String {
+    let match = path.prefixMatch(of: legacyExternalGeneratedFile) ?? path.prefixMatch(of: legacyExternalFile)
+    return match.map { String($0.1) } ?? ""
+}
 
 // MARK: Runfiles Paths Computation